QHDM Vol1 Part03 RoadwayDesignElements OctFinal

Volume 1

Part 3 Roadway Design Elements

VOLUME1PART3ROADWAYDESIGNELEMENTS

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VOLUME1

Disclaimer

The StateofQatarMinistryof Transport (MOT)provides access to theQatarHighwayDesignManual (QHDM) andQatar TrafficControlManual (QTCM)on theweb and ashard copies asVersion(1.0)ofthesemanuals,withoutanyminimumliabilitytoMOT.Under no circumstances doesMOTwarrantor certify the information tobe freeof errorsordeficienciesofanykind.Theuseofthesemanualsforanyworkdoesnotrelievetheuserfromexercisingduediligenceandsound engineering practice, nor does it entitle the user to claim or receive any kind ofcompensationfordamagesorlossthatmightbeattributedtosuchuse.AnyfuturechangesandamendmentswillbemadeavailableontheMOTwebsite.Usersofthesemanualsshouldcheckthattheyhavethemostcurrentversion.Note:Newfindings,technologies,andtopicsrelatedtotransportationplanning,design,operation,andmaintenancewillbeusedbyMOTtoupdatethemanuals.Usersareencouragedtoprovidefeedback through the MOT website within a year of publishing the manuals, which will bereviewed,assessed,andpossiblyincludedinthenextversion.Copyright2015.Allrightsreserved.

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VOLUME 1 PART 3 ROADWAY DESIGN ELEMENTS

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VOLUME 1 PAGE I

Contents Page

Acronyms and Abbreviations ................................................................................................... xi

1 Introduction .................................................................................................................... 1 1.1 Design Speed ............................................................................................................... 1

1.1.1 Posted Speed ................................................................................................ 2 1.2 Design Speed Related Parameters .............................................................................. 3

1.2.1 Changeover of Design Speed ....................................................................... 3 1.2.2 Reconstruction and Connection to Existing Roads ...................................... 4 1.2.3 Departures from Standards .......................................................................... 4 1.2.4 Special Considerations ................................................................................. 7

1.3 Sustainability ............................................................................................................... 7

2 Sight Distance ............................................................................................................... 11 2.1 Basic Types of Sight Distance ..................................................................................... 11 2.2 Stopping Sight Distance ............................................................................................. 12

2.2.1 SSD Model and Parameters ........................................................................ 12 2.2.2 Stopping Sight Distance Design Values ...................................................... 12 2.2.3 Horizontal Restrictions to Stopping Sight Distance .................................... 14 2.2.4 Vertical Restrictions to Stopping Sight Distance ........................................ 16

2.3 Passing Sight Distance ............................................................................................... 17 2.4 Decision Sight Distance .............................................................................................. 19 2.5 Intersection Sight Distance ........................................................................................ 21

2.5.1 Case A: Intersections with No Control ....................................................... 22 2.5.2 Case B: Intersections with Stop Control on Minor Road ............................ 24 2.5.3 Case C: Intersections with Yield Control on Minor Road ........................... 28 2.5.4 Case D: Intersections with Traffic Signal Control ....................................... 31 2.5.5 Case E: Intersections with All-Way Stop Control ....................................... 32 2.5.6 Case F: Left Turns from Major Road ........................................................... 32

2.6 Special Considerations ............................................................................................... 33 2.7 Departures ................................................................................................................. 34

3 Horizontal Alignment .................................................................................................... 35 3.1 Simple Horizontal Curve and Spirals .......................................................................... 35 3.2 Alignment combinations using simple curves and tangents ..................................... 37 3.3 General Design Considerations ................................................................................. 40

3.3.1 Maximum Centerline Deflection without a Horizontal Curve.................... 40 3.3.2 Minimum Curve Lengths ............................................................................ 41

3.4 Cross Slope and Superelevation ................................................................................ 41 3.5 Superelevation Transition and Spirals ....................................................................... 43

3.5.1 Axis of Rotation .......................................................................................... 44 3.5.2 Rate of Rotation ......................................................................................... 44 3.5.3 Calculation of Superelevation Transition Lengths ..................................... 47 3.5.4 Spirals ......................................................................................................... 49 3.5.5 Positioning of Superelevation Transition ................................................... 50


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3.5.6 Superelevation on Compound Curves ........................................................ 52 3.5.7 Superelevation on Reverse Curves ............................................................. 53 3.5.8 Shoulder Slopes on Superelevated Roadways ........................................... 54

3.6 Traveled Way Widening on Horizontal Curves .......................................................... 58 3.7 Horizontal Clearance or Lateral Offset ...................................................................... 61 3.8 Widths for Turning Roadways at Intersections ......................................................... 62 3.9 Special Considerations ............................................................................................... 64

4 Vertical Alignment ........................................................................................................ 65 4.1 Terrain ........................................................................................................................ 66 4.2 Longitudinal Grades ................................................................................................... 66

4.2.1 Maximum Longitudinal Grades .................................................................. 66 4.2.2 Minimum Grades ........................................................................................ 66 4.2.3 Minor Road Grades at Intersections .......................................................... 66

4.3 Vertical Curves ........................................................................................................... 67 4.3.1 Crest Vertical Curves .................................................................................. 67 4.3.2 Sag Vertical Curves ..................................................................................... 67 4.3.3 Crest Vertical Curve Design ........................................................................ 69 4.3.4 Sag Vertical Curve Design ........................................................................... 71 4.3.5 Minimum Length of Vertical Curves ........................................................... 74 4.3.6 Maximum Grade Change without a Vertical Curve .................................... 74

4.4 Vertical Clearances .................................................................................................... 76 4.5 Special Considerations ............................................................................................... 78

5 General Considerations ................................................................................................ 79 5.1 General ....................................................................................................................... 79 5.2 Harmonizing the Horizontal Alignment ..................................................................... 79 5.3 Harmonizing the Vertical Alignment .......................................................................... 81 5.4 Phasing of Horizontal and Vertical Alignments ......................................................... 82 5.5 Alignment Coordination in Design ............................................................................. 83

6 Cross Section Elements ................................................................................................. 89 6.1 General Considerations ............................................................................................. 89

6.1.1 Introduction ................................................................................................ 89 6.1.2 Design Principles ......................................................................................... 90 6.1.3 Road Network Objectives ........................................................................... 91 6.1.4 Departures .................................................................................................. 93

6.2 Design Requirements ................................................................................................. 93 6.2.1 Travel Lanes ................................................................................................ 94 6.2.2 Shoulders .................................................................................................... 95 6.2.3 Hard Strips .................................................................................................. 97 6.2.4 Auxiliary Lanes ............................................................................................ 97 6.2.5 Medians ...................................................................................................... 98 6.2.6 Service Roads ............................................................................................ 102 6.2.7 Frontage Roads ......................................................................................... 102 6.2.8 Parking Bays and Lanes ............................................................................ 105


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6.2.9 Parallel Roadside Parking ......................................................................... 106 6.2.10 Angled Roadside Parking .......................................................................... 106 6.2.11 Off Street Parking ..................................................................................... 107 6.2.12 Off Street Disabled Parking ...................................................................... 110 6.2.13 Curbs ........................................................................................................ 110 6.2.14 Road Restraint System ............................................................................. 114 6.2.15 Side Slopes ............................................................................................... 114 6.2.16 Clearances ................................................................................................ 115 6.2.17 Fencing ..................................................................................................... 116 6.2.18 Roadside Elements and Verges ................................................................ 116 6.2.19 Pedestrian Facilities.................................................................................. 118 6.2.20 Bike Facilities ............................................................................................ 118 6.2.21 Utilities ..................................................................................................... 118 6.2.22 Right-of-Way ............................................................................................ 118 6.2.23 Typical Cross Sections .............................................................................. 119

7 Integrated Road and Landscape Design ....................................................................... 159 7.1 Integrated Road Design Principles ........................................................................... 160 7.2 Road Siting and Alignment ...................................................................................... 160

7.2.1 Earthworks ............................................................................................... 160 7.2.2 Retaining Walls ......................................................................................... 161 7.2.3 Rock Cut ................................................................................................... 161 7.2.4 Interchanges and Roundabouts ............................................................... 162 7.2.5 Gateways .................................................................................................. 162 7.2.6 Special Areas ............................................................................................ 163

7.3 Structures ................................................................................................................ 163 7.3.1 Bridges ...................................................................................................... 163 7.3.2 Pedestrian Bridges ................................................................................... 164 7.3.3 Tunnels ..................................................................................................... 165 7.3.4 Pedestrian Underpasses ........................................................................... 165

7.4 Fences and Walls ..................................................................................................... 166 7.5 Water Management/ and Conservation ................................................................. 166

7.5.1 Detention Systems ................................................................................... 167 7.5.2 Infiltration Systems .................................................................................. 167

7.6 Landscape and Utilities ............................................................................................ 168 7.7 Sustainable Landscape Design ................................................................................. 168

7.7.1 Introduction ............................................................................................. 168 7.7.2 Urban Street and Landscape Assessment and Planning .......................... 169 7.7.3 Soils .......................................................................................................... 169 7.7.4 Water Conservation ................................................................................. 170 7.7.5 Plant Species ............................................................................................ 170 7.7.6 Materials .................................................................................................. 170 7.7.7 Landscape Maintenance and Management ............................................. 171


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8 Other Elements Affecting Design ................................................................................ 173 8.1 Erosion Control ........................................................................................................ 173 8.2 Rest Areas ................................................................................................................ 173

8.2.1 Spacing of Rest Areas ............................................................................... 173 8.2.2 Site Selection ............................................................................................ 173 8.2.3 Rest Area Design ....................................................................................... 174

8.3 Traffic Control Devices ............................................................................................. 180 8.3.1 Signing and Marking ................................................................................. 180 8.3.2 Traffic Signals ............................................................................................ 181

8.4 Noise Barriers ........................................................................................................... 181 8.4.1 Placement ................................................................................................. 181

8.5 Fencing ..................................................................................................................... 182 8.6 Sand Abatement in Dune Areas ............................................................................... 182

References ........................................................................................................................... 185


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Tables

Table 1.1 Design and Posted Speeds for Various Road Classifications ............................... 2

Table 1.2 Design Speed Related Parameters ...................................................................... 3

Table 2.1 Stopping Sight Distance for Level Roadways with Grades less than 3 Percent ................................................................................................... 13

Table 2.2 Passing Sight Distance for Two-Lane Roadways ............................................... 18

Table 2.3 Decision Sight Distance ..................................................................................... 21

Table 2.4 Length of the Sight Triangle Legs, for Intersections with No Control ............... 23

Table 2.5 Adjustment Factors for Intersection Sight Distance Based on Approach Grade ................................................................................................ 24

Table 2.6 Time GapCase B1, Left Turn from Stop .......................................................... 25

Table 2.7 Intersection Sight DistanceCase B1, Left Turn from Stop .............................. 26

Table 2.8 Time GapCase B2, Right Turn from Stop and Case B3, Crossing Maneuver .. 27

Table 2.9 Intersection Sight DistanceCase B2, Right Turn from Stop and Case B3, Crossing Maneuver ............................................................................................ 27

Table 2.10 Crossing Maneuver from Yield Controlled Approaches, Length of Minor Leg and Travel Time from the Decision Point ................................................... 29

Table 2.11 Length of Sight Triangles along Major RoadCase C1, Crossing Maneuver from Yield Controlled Intersections ................................................. 30

Table 2.12 Gap Acceptance Time for Left- and Right-Turn Maneuvers from Yield-Controlled Intersections ........................................................................... 31

Table 2.13 Intersection Sight Distance along Major RoadCase C2, Left or Right Turn at Yield-Controlled Intersections ..................................................... 31

Table 2.14 Time Gap for Case F Left Turn from the Major Road ........................................ 33

Table 2.15 Intersection Sight DistanceCase F, Left Turn from the Major Road .............. 33

Table 3.1 Minimum Radius without Superelevation ........................................................ 42

Table 3.2 Superelevation for Radii and Design Speed (percent) ...................................... 43

Table 3.3 Maximum Relative Gradients ............................................................................ 45

Table 3.4 Adjustment Factors for Number of Lanes Rotated ........................................... 48

Table 3.5 Traveled Way Widening Criteria on Horizontal Curves ..................................... 58

Table 3.6 Traveled Way Widening Criteria at Horizontal Curves, Inside Curve Radius less 100 m or Less .................................................................................. 59

Table 3.7 Design Widths of Pavements for Turning Roadways ........................................ 63

Table 4.1 Maximum Grades .............................................................................................. 66

Table 4.2 Design Controls for Crest Vertical Curve Design Based on Stopping Sight Distance ............................................................................................................. 71

Table 4.3 Design Controls for Sag Vertical Curves .............................................................. 74


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Table 4.4 Maximum Grade Change without a Vertical Curve ........................................... 75

Table 4.5 Vertical Clearance at Structures ........................................................................ 76

Table 4.6 Sag Radius Compensation ................................................................................. 77

Table 6.1 Mainline Rate of Change of Width for a Standard Lane .................................... 95

Table 6.2 Typical Roadway Cross Sections ........................................................................ 96

Table 6.3 Minimum Median Width ................................................................................... 99

Table 6.4 Roadside Angled Parking Dimensions for One-Way Operation ...................... 107

Table 6.5 Parking Bay Dimensions .................................................................................. 109

Table 6.6 Typical Cross Sections Urban Roads ............................................................. 120

Table 6.7 Typical Cross Sections Rural Roads ............................................................... 121

Table 8.1 Demand Assessment Guidelines for Provision ................................................ 179

Table 8.2 Minimum Parking Provision ............................................................................ 179

Figures

Figure 2.1 Horizontal Stopping Sight Distance ................................................................... 15

Figure 2.2 Stopping Sight Distance at Crest of Vertical Curve ............................................ 16

Figure 2.3 Stopping Sight Distance at Sag Vertical Curve ................................................... 17

Figure 2.4 Passing Maneuver .............................................................................................. 17

Figure 2.5 Sight Triangles (Uncontrolled and Yield Controlled) ......................................... 23

Figure 2.6 Sight Triangles (Stop Controlled) ....................................................................... 25

Figure 2.7 Left Turns from Major Roads ............................................................................. 32

Figure 3.1 Simple Curve Elements ...................................................................................... 36

Figure 3.2 Simple Curve with Spirals .................................................................................. 37

Figure 3.3 Compound Curve ............................................................................................... 38

Figure 3.4 Broken Back Curve ............................................................................................. 39

Figure 3.5 Reverse Curve .................................................................................................... 40

Figure 3.6 Development of Superelevation ........................................................................ 46

Figure 3.7 Number of Lanes Rotated for Undivided Roadways ......................................... 48

Figure 3.8 Superelevation Transition for Two-Lane Roadways .......................................... 51

Figure 3.9 Superelevation Transition on Compound Curves (Distance between PC and PCC is less than or equal to 90 m) .............................................................. 52

Figure 3.10 Superelevation Transition on Compound Curves (Distance between PC and PCC is greater than 90 m) ........................................................................... 53

Figure 3.11 Superelevation between Reverse Curves .......................................................... 54


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Figure 3.12 Superelevation Development for Freeways and Expressways ......................... 55

Figure 3.13 Superelevation Development for Arterial and Collectors with Narrow Medians ................................................................................................ 56

Figure 3.14 Superelevation Development for Freeway and Expressways (Shoulder Break Option) .................................................................................... 57

Figure 3.15 Travel Lane Widening on Horizontal Curves ..................................................... 60

Figure 3.16 Horizontal Clearance or Lateral Offset .............................................................. 61

Figure 3.17 Turning Roadway Widths on Curves at Intersections ....................................... 63

Figure 4.1 Minor Road Vertical Alignment Approach at Intersections .............................. 67

Figure 4.2 Vertical Curve Elements ..................................................................................... 68

Figure 4.3 Stopping Sight Distance at Crest of Vertical Curve ........................................... 70

Figure 4.4 Stopping Sight Distance at Sag Vertical Curve .................................................... 72

Figure 4.5 Additional Clearances at Underpasses for Sag Vertical Curves ......................... 77

Figure 5.1 Example of a Kink and Improvement with Larger Radius ................................. 80

Figure 5.2 Alignment Relationships in Roadway Design1 of 4 ....................................... 84




Figure 6.1 Cross Section Design Flow Chart ....................................................................... 92

Figure 6.2 Typical Median Layouts ................................................................................... 101

Figure 6.3 Typical Frontage Road Arrangements1 of 2 ................................................ 103

Figure 6.4 Typical Frontage Road Arrangements2 of 2 ................................................ 104

Figure 6.5 Minimum Clearance of Parking Lane from Intersection ................................. 105

Figure 6.6 Roadside Parking Parallel Bay Dimensions ...................................................... 106

Figure 6.7 Roadside Parking in Angled Bay Layout .......................................................... 107

Figure 6.8 Parking Bay Dimensions .................................................................................. 108

Figure 6.9 Standard Curb Types ....................................................................................... 111

Figure 6.10 Typical Urban Local Access, One-way System - 10 m Right-of-Way (Residential) ..................................................................................................... 122

Figure 6.11 Typical Urban Local Access - 12 m Right-of-Way (Residential) ....................... 123



Figure 6.14 Typical Urban Local Access - 20 m Right-of-Way (Commercial) ...................... 126

Figure 6.15 Typical Urban Local Access - 20 m Right-of-Way (Industrial) .......................... 127

Figure 6.16 Typical Urban Minor Collector - 20 m Right-of-Way (Residential) .................. 128

Figure 6.17 Typical Urban Minor Collector - 20 m Right-of-Way (Commercial) ................ 129


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Figure 6.18 Typical Urban Minor Collector - 20 m Right-of-Way (Industrial)..................... 130


Figure 6.20 Typical Urban Minor Collector - 24 m Right-of-Way (Commercial) ................ 132

Figure 6.21 Typical Urban Minor Collector - 24 m Right-of-Way (Industrial)..................... 133


Figure 6.23 Typical Urban Major Collector - 32 m Right-of-Way (Residential) with Service Road ............................................................................................ 135

Figure 6.24 Typical Urban Major Collector - 32 m Right-of-Way (Residential) with Service Road ............................................................................................ 136

Figure 6.25 Typical Urban Major Collector - 40 m Right-of-Way (Commercial) with Service Road ............................................................................................ 137

Figure 6.26 Typical Urban Major Collector - 40 m Right-of-Way (Commercial) with Service Road ............................................................................................ 138

Figure 6.27 Typical Urban Major Collector - 40 m Right-of-Way (Industrial) with Service Road ............................................................................................ 139

Figure 6.28 Typical Urban Major Collector/Minor Arterial - 40 m Right-of-Way (Industrial) ....................................................................................................... 140

Figure 6.29 Typical Urban Minor Arterial - 50 m Right-of-Way (Commercial/Industrial) .................................................................................. 141

Figure 6.30 Typical Urban Minor Arterial - 50 m Right-of-Way (Commercial/Industrial) with Service Road ..................................................... 142

Figure 6.31 Typical Urban Major Arterial - 50 m Right-of-Way (Commercial/Industrial) with Service Road ..................................................... 143

Figure 6.32 Typical Urban Minor/Major Arterial - 50 m Right-of-Way (Commercial/Industrial) with Service Road ..................................................... 144

Figure 6.33 Typical Urban Major Arterial - 64 m Right-of-Way (Commercial/Industrial) .................................................................................. 145

Figure 6.34 Typical Urban Boulevard - 64 m Right-of-Way (Recreational) ........................ 146

Figure 6.35 Typical Urban Boulevard64 m Right-of-Way (Recreational) ........................ 147

Figure 6.36 Typical Urban Boulevard 64 m Right-of-Way (Commercial) ........................ 148

Figure 6.37 Typical Urban Expressway 64 m Right-of-Way (6-Lane Divided Highway) ................................................................................ 149

Figure 6.38 Typical Urban Expressway 64 m Right-of-Way (8-Lane Divided Highway) ................................................................................ 150

Figure 6.39 Typical Urban Expressway 264 m Right-of-Way (8-Lane Divided Highway-with Frontage Road) ............................................... 151

Figure 6.40 Typical Rural Access Road 16 m to 20 m Right-of-Way ................................ 152

Figure 6.41 Typical Rural Collector 24 m to 40 m Right-of-Way ..................................... 153

Figure 6.42 Typical Rural Arterial 64 m Right-of-Way ..................................................... 154


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Figure 6.43 Typical Rural Freeway 264 m Right-of-Way (with Frontage Road) .............. 155

Figure 6.44 Typical Embankment Cross Sections ............................................................... 156

Figure 6.45 Typical Cut Cross Sections ............................................................................... 157

Figure 6.46 Requirement for Barriers on Embankments ................................................... 158

Figure 8.1 Typical Rest Area Plan ..................................................................................... 174

Figure 8.2 Disabled Parking at Rest Areas ........................................................................ 176

Figure 8.3 Bus Parking Details at Rest Areas .................................................................... 177

Figure 8.4 Truck Parking Details at Rest Areas ................................................................. 178


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Acronyms and Abbreviations

AASHTO American Association of State Highway and Transportation Officials

Departure Departure from Standards

DMRB Design Manual for Roads and Bridges

DSD decision sight distance

HSSD horizontal stopping sight distance

HOV high-occupancy vehicle

ISD intersection sight distance

kph kilometers per hour

m meter(s)

MASH Manual for Assessing Safety Hardware

NMU nonmotorized users, such as pedestrians, cyclists, and equestrians

OHPS over-height protection system

PC point of curvature

PCC point of compound curvature

PSD passing sight distance

QHDM Qatar Highway Design Manual

QTCM Qatar Traffic Control Manual

sec seconds

VSSD vertical stopping sight distance


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1 Introduction

This part outlines the following design controls and elements to be applied in the design of the roadway geometry:

Design speed Roadway cross sections Sight distances Horizontal alignment Vertical alignment Grades and cross slope Vertical clearances

The roadway design process begins with the establishment of basic design controls and design criteria, the most important of these being design speed. The design process proceeds sequentially, with establishment of the basic typical cross section, followed by the setting of the horizontal alignment and then the vertical alignment.

Roadway design is not performed on a blank sheet of paper, nor is it an automatic or rote exercise. The context, i.e., the terrain, adjacent land use, and location-specific features or constraints, influences the design. Good design is necessarily iterative. There is always more than one reasonable solution to a design problem. The role of the designer as an engineering professional should be to seek the highest value solution; with value being dependent on the site-specific conditions and stakeholder input.

The unique engineering challenges designers face involve the often conflicting transportation values of mobility and safety. Traditionally, design focus has been on meeting the desires of road users to minimize their travel time. This is accomplished by designing the road for the highest speed that is reasonable given the context. Designing for high speeds, however, presents challenges because human driving capabilities are limited at high speeds, and the consequences of human error are heightened because the severity of crashes is significantly greater at higher speeds.

1.1 Design Speed The design speed for a road or highway is a selected speed that is used to determine the dimensions, values, and characteristics of the roadway. A fundamental principle of design speed is that it applies over relatively long sections of roadway. The professional designer selects a speed that is logical and reasonable using the guidance provided by the Qatar Highway Design Manual (QHDM). The selected speed should reflect the anticipated and desired operating speed, the topography, the adjacent land use, and the intended primary function of the highway as determined by its functional


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classification. The selection of a design speed should be made with an awareness of the need to achieve safety, mobility, and efficiency within the constraints of environmental quality, economics, aesthetics, and social and political impacts. In selecting a design speed, the designer is setting the basis by which all of the basic elementscross section and alignmentwill be established.

Meeting the expectations of drivers should influence the selection of a design speed. Where the reasons for lower speed are obvious, such as the land use or terrain, drivers are apt to expect and accept lower speed. Drivers do not adjust their speeds to the importance of the highway, but rather in response to their perception of the physical limitations of the highway and its traffic.

A related control in highway design is the setting of legal operation of vehicles through the posting of speed limits. By policy in Qatar, the posted speed limits that apply are related to the design speeds, as shown in Table 1.1. The design speeds and corresponding posted speeds vary by functional classification and land use frontage.

1.1.1 Posted Speed Posted speed is the mandatory speed limit applied to a road. The speed limit is displayed on the roadside and is enforceable. Table 1.1 lists the posted speed limits to be implemented in relation to design speed.

Table 1.1 Design and Posted Speeds for Various Road Classifications

Road Classification Land Use Frontage Design

Speeds (kph) Posted Traffic Speeds (kph)

Urban Expressway Commercial, industrial, recreational, park 100 /120/ 140 80/100/120

Urban ArterialMajor Commercial or industrial land use preferred 50/80/ 100

50/60/80

Urban ArterialMinor Commercial or industrial land use preferred 50/80/100

50/60/80

ArterialBoulevard Retail or commercial 50/80/100 50/60/80

Urban CollectorMajor

Industrial 50/80/100 50/60/80

Commercial 50/80/100 50/60/80

Urban CollectorMinor

Industrial or commercial 50 50

Residential 50 50

Recreational 50 50

Urban Local Road Industrial 50 50

Commercial, residential, or recreational 30/40/50 30/40/50

Rural Freeway Not applicable 100/120/140 80/100/120

Rural Arterial Not applicable 80/100/120 60/80/100

Rural Collector Not applicable 50/80/100 50/60/80

Rural Local Not applicable 50/80 50/60

Posted speeds are generally lower than design speeds for roadways with design speeds greater than 50 kilometers per hour (kph). Posted speed for roadways with design speeds of 80 kph or greater is 20 kph lower; for roadways with design speeds less than 80 kph and greater than 50 kph posted speed is 10 kph lower. Design and posted speeds are the


VOLUME 1 PAGE 3

same for roadways with design speeds of 50 kph or lower. The road classifications are defined in Part 2, Planning.

Selection of a design speed is one of the very first project decisions. Selection of a design speed outside the values shown in Table 1.1 shall constitute a Departure from Standards (Departure). Should a Departure be considered, the process of evaluation and approval should occur before proceeding with any engineering design. The selection of design speed shall be approved by the Overseeing Organization. Refer to Part 25, Departures from Standards Process, for more information.

Refer to Section 1.2.4 for design speeds for roads that require special consideration. They should be agreed to with the Overseeing Organization.

1.2 Design Speed Related Parameters The design of most elements of the roadway are influenced by the selected design speed. Table 1.2 details the main design speed related parameters dealt with in greater depth in their respective clauses in this part.

Table 1.2 Design Speed Related Parameters

Parameter Reference

Stopping sight distance Chapter 2: Section 2.2

Passing sight distance Chapter 2: Section 2.3

Decision sight distance Chapter 2: Section 2.4

Intersection sight distance Chapter 2, Section 2.5

Horizontal curvature Chapter 3, Section 3.1

Vertical curvature Chapter 4, Section 4.3

Lane widths Chapter 6: Section 6.2

1.2.1 Changeover of Design Speed Transitions between roads (or sections of a road) with different design speeds shall be implemented so as not to present the driver with an abrupt change in the roadways characteristics or appearance. A change in the design speed should not exceed 20 kph. For example, in transitioning to a lower design speed from 100 kph the new, lower design speed should not be less than 80 kph.

At the interface between sections of roadway designed to different design speeds, designers check that the curvature and sight distance is adequate for the approach design speed. See Chapter 3 regarding transition curves where the road passes through an area where the curve radius must be reduced beyond the limiting radius to accommodate design speed.


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Designers should avoid interfaces between different design speeds near horizontal or vertical curvature less than the requirements for the higher design speed, and at or near steep vertical grades. Sufficient warning signs should be provided in advance of reaching the section of the road with lower speed. For details of signing the speed control refer to Part 2, Planning.

1.2.2 Reconstruction and Connection to Existing Roads Care shall be taken in the design of road to be reconstructed, with such reconstruction resulting in a significant change in the roadway geometry. At the project limits, the design characteristics transition from the newly reconstructed road to the existing road to remain in place.

The setting of appropriate project limits is one measure of care. Logical limits may include an intersection, or a tangent highway section at which the vertical alignment provides stopping sight distance that is greater than the minimum. Careful consideration should be given for roads passing between rural and urban areas, posted speed step down and also two lane to single lane roadways, although this latter case should be limited to intersection locations only.

Clear signing is needed at all locations where there is a speed reduction.

1.2.3 Departures from Standards The standards herein represent the various criteria and maximum/minimum levels of provision whose incorporation in the road design would achieve an acceptable level of transportation performance. In most cases, designs can be achieved that do not require the use of the lowest levels of given design parameters. At some locations on new roads or reconstruction of existing roads it may not be possible to provide even the lowest levels of design parameters in economic or environmental terms because of unique context features including existing landmarks, high-rises; religious, cultural, and historic sites; natural preserves; and community resources. Prevailing circumstances may identify sufficient advantages that may justify a Departure from Standards. The parameters are not to be regarded as fixed in all circumstances. Departures should be considered and assessed in terms of their effects on the economic worth of the project, the environment, and the safety of road users. Designers should always have regard to the cost-effectiveness of the design provision. The implications, particularly in relation to safety risk, should be quantified to the extent possible. Engineering judgment is necessary in many cases. In exercising judgment, professional designers considering a Departure should have knowledge of the operational and safety effects of roadway design features. Part 25, Departures from Standards Process, provides technical guidance including research references that designers can review to enable appropriate judgments. The QHDM covers a wide range of geometric elements and design dimensions. Based on a review of international, peer-reviewed research and practices of major national highway agencies, the following 16 criteria have been identified as being of sufficient importance that the inability, for whatever reason, to meet the minimum design value shall require a formal Departure from Standards:


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1. Design speed

2. Lane width

3. Shoulder width

4. Bridge width

5. Horizontal alignment

6. Superelevation

7. Vertical alignment

8. Longitudinal grade

9. Sight distance

10. Cross slope

11. Vertical clearance

12. Lateral offset to obstruction

13. Structural capacity (not a geometric element)

14. Acceleration and deceleration lane lengths on fully access controlled highways

15. Weaving section operations on fully access controlled highways

16. Bike path or shared use path

The Overseeing Organization requires a formal written Departure Application for each design exception if the specified maximum or minimum criteria related to the 16 controlling criteria above are not met. The process for submitting a written Departure Application is given in Part 25, Departures from Standards Process.

Other design parameters, values, and policies in QHDM are guidelines to promote consistency in design and achieve overall quality control in the project development process. The Overseeing Organization provides oversight on all design and specification aspects of all projects. Designers should strive to fully apply all published criteria, regardless of whether they are subject to the Departure process.

This section introduces the concept of a hierarchy of permitted values for geometric layout parameters such as sight distance, horizontal and vertical curves. That hierarchy is based upon minimum standards based on design speed. Values equal to or greater than the minimum results in safer alignments and minimizes journey times. The hierarchy of values enables a flexible approach to be applied where the strict application of minimum requirements would lead to disproportionately high construction costs or severe environmental impact upon people, properties, and landscapes. Successive levels in the hierarchy invoke more stringent consideration in line with the need to carefully consider safety.


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Over the years, there have been many advances in road layout design and associated improvements for the assessment of safety and operational aspects. Research has strengthened the understanding of driver behavior, and safety audits and other initiatives in the mechanics of assessing and checking scheme layouts have made the design process more rigorous and reliable.

Experience in the application of the hierarchy of values indicates that the environmental and financial benefits gained from this increased flexibility can be considerable. The designer must carefully consider the benefits and any potential disadvantages of Departures. Additional guidance in Part 25, Departures from Standards Process, describes the approach to be taken to assessing Departures. Any such Departures must be agreed to in writing with the Overseeing Organization. Part 25 gives a procedure for the preparation and submission requirements for Departures.

Principles to follow when preparing options that include Departures are listed below. It is likewise a list of factors to be taken into account when considering the merits of options. Designers should consider whether and to what degree the site is:

Isolated from other Departures

Isolated from intersections

One where drivers are provided minimum stopping sight distance

Subject to momentary visibility impairment only

One that would affect only a small proportion of the traffic

On straightforward geometry readily understandable to drivers

On a road with no frontage access

One where traffic speeds would be reduced locally by adjacent speed limits or road geometry, such as uphill sections, approaching roundabouts, major/minor intersections where traffic has to yield or stop, and so on

Designers should consider whether the following should be introduced in conjunction with any Departure:

Crash prevention measures; for safety fencing, increased skidding resistance

Warning signs and road markings to alert the driver to the layout ahead

Designers should have regard for the traffic flows carried by the link. High flows may carry a greater risk of queues and standing traffic approaching intersections in the peak period. Conversely, lower flows might encourage higher speeds.

The road classifications for Qatar are described in Part 2, Planning. The selection of a design speed is difficult for some roads in the older areas of the city. Those areas are not so easily classified into land use, and factors such as access and parking must be assessed in determining the design speed. Other considerations are the number and


VOLUME 1 PAGE 7

spacing of intersections on a particular section of road. Departures provide a means of accommodating these areas.

Departures from minimum or maximum values specified for the 16 controlling criteria may be considered when context, cost, or environmental savings are considered to be significant, except in the following circumstances:

Immediately following an overtaking section on undivided roads

On the immediate approach to an intersection, other than a roundabout, where frequent turning traffic will occur.

1.2.4 Special Considerations The posted speed for local roads in residential, commercial, and recreational areas is 50 kph but may be reduced to 30 kph in areas of high pedestrian activity, or where the local roads are provided for access only. Part 2, Planning, identifies circumstances on lower category urban roads where the 30 kph posted speed should be applied.

The lower design speeds of 50 kph and 30 kph applied in urban areas do not require transitions or superelevation on bends. Refer to Section 3.3 and Section 3.4.

One-way roads may be used on local roads for access, usually in the form of discrete loops. One-way roads should be designed so as not to encourage speeding. This may be achieved by the use of narrower roadway lanes, avoiding long tangent sections of road, and implementing traffic calming measures. Refer to Part 23, Design and Operations for Road Safety.

Care shall be taken to ensure that traffic calming measures being introduced do not impede emergency service vehicles.

1.3 Sustainability The key sustainability principles for highway design in Qatar are summarized in Part 1, Introduction to QHDM and Guidance, and Part 21, Environmental. Early consideration of potential impacts and how the design can help to avoid or minimize them is a key principle to be followed. Sustainable design should minimize the need for design rework and achieve optimal economic feasibility considering costs over the whole asset life, including costs that may be incurred from changes to traffic volumes, urbanization, user types, and environmental conditions. The design should consider, from the earliest stages, opportunities to design out adverse environmental and social impacts and how enhancements can be incorporated. Regarding the design of roadways, roadway elements, and intersections, designers should consider the following issues. See also Part 21, Environmental.


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Water use: Designers should consider the requirements for water use during construction and how designs can be optimized to avoid or minimize the need for water.

Soil erosion and contamination: The design should minimize soil erosion, windblown sand, and contamination during construction and operation. Where possible, the road should avoid areas likely to lead to erosion and contamination impacts, such as sabkha, sand dunes, and contaminated land. Natural vegetation should be used as a natural barrier to sand movement where possible.

Material use and resource efficiency: Designs should minimize the quantity of raw materials required and when specifying materials or setting technical specifications. Designers should give due consideration to incorporating sustainable materials into their designs, such as locally sourced, reused, or recycled materials, or low embodied energy/ carbon materials. The design should be optimized to minimize waste during construction and maintenance.

Climate change adaptation: Designers should ensure that consideration is given to the potential impacts of climate change on roadways and that resilience is built into their design for issues such as increased temperatures, rainfall intensity, sea level rise, and erosion control.

Provision for sustainable transport: Designers should consider the needs of and make suitable provisions for both nonmotorized and public transport users in terms of accessing the road network crossing the roadway, in order to avoid severance impacts. Where roadways are unsuitable for nonmotorized users, the design should consider incorporating segregated pathways for bicycle users. Intersections should be made safe and usable to nonmotorized users to avoid severance.

Air quality: Designers should take measures to ensure that operational air quality impacts are minimized through design. Road alignment, traffic management to control speed and to encourage specific traffic behavior, landscaping, gradients, cross-slope roads, and corner angles affecting speeds and flows should all be considered. Roadways should be integrated with the public transport network, and pedestrian and bike access should be provided.

Noise: Designers should consider routing alignments to maximize the distance between receptors and the roadway. Where sensitive receptors are affected by a roadway, the design should incorporate adequate mitigation measures to reduce traffic noise through the specification of low-noise surfacing materials or of environmental barriers.

Visual impact and landscape design: Landscape design should minimize the visual impact of the road and seek to enhance the visual amenity value of the area where possible.

Ecology and biodiversity: Roadway design should minimize ecological and biodiversity effects within the roadway corridor and optimize opportunities for enhancement through landscaping or planting strategies for example. Severance of wildlife corridors and fragmentation should be mitigated through design


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Operational energy use: Energy-consuming equipment required to fulfil a function of the roadway should be specified to maximize energy efficiency in use. Designers should incorporate opportunities for micro-renewable energy generation co-located with equipment requiring an energy demand.

Safety: It is important to ensure that design characteristics, such as stopping sight distance, curve radius, lane width, and superelevation, are commensurate with the speed limit of the road. The characteristics of the road determine the safety of the speed limit and the degree to which road users will accept the speed limit. See Part 23, Design and Operation of Road Safety.

Archaeology and cultural heritage: Roadway design should optimize the protection, preservation, and enhancement of sites of archaeological or cultural value by routing so as to avoid them where possible and to optimize horizontal and vertical alignments in order to avoid or minimize severance and visual and noise intrusion impacts.


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SECTION [?]

[TITLE]

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2 Sight Distance

A fundamental principle of road design is that the driver should be able to see the roadway and its environment sufficiently in advance of the vehicle such that the driver can maneuver and/or change speeds while maintaining full control of the vehicle. The term sight distance expresses this fundamental principle. Sight distance is the continuous length of the roadway ahead that is visible to the driver. Drivers should be able to see far enough ahead to safely and efficiently perform any carry out any legal maneuvers. They should have sufficient view of the road to safely avoid conflicts that may occur.

2.1 Basic Types of Sight Distance The four important maneuvers drivers undertake form the basis for the design for sight distance in Qatar:

1. Stopping Sight Distance: applicable on all roadways

2. Passing Sight Distance: applicable on two lane undivided roadways

3. Decision Sight Distance: applicable on urban and rural roads where road users have to make complex decisions, for example exiting at interchanges

4. Intersection Sight Distance: applicable at all intersections

Simple operational models are used to compute design values for each type of sight distance. Each model employs assumptions for the basic parameters describing the assumed location of a drivers eye, and the assumed type and location of an object or feature that characterizes or controls the design maneuver.

The criteria and models applied to roads in Qatar are based on the latest research (National Cooperative Highway Research Program [NCHRP] Report 383, 1996 and NCHRP Report 400, Fambro et al., 1997) and its application to design criteria in other national design manuals and policies, such as the Design Manual for Roads and Bridges (DMRB; Department for Transport, 2002).

Two critical features to be considered while evaluating sight distances are object height and eye height. Eye height is 1.08 m for all sight distances. Object height is 0.6 m for stopping and decision sight distance and 1.08 m for intersection and passing sight distance. These heights are in reference to the passenger cars. For large trucks eye height varies from 1.8 m to 2.4 m with a recommended value of 2.3 m.


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2.2 Stopping Sight Distance Roads should be designed such that a driver operating a passenger car proceeding at design speed should be able to perceive an object of sufficient size to represent a risk if struck, and brake to a full stop in a reasonable manner thus avoiding collision with the object. This statement expresses what is referred to as stopping sight distance (SSD).

2.2.1 SSD Model and Parameters The SSD model is based on human factors research and the capabilities of vehicles. The following parameters are used (American Association of State Highway and Transportation Officials [AASHTO], 2011a):

Height of drivers eye 1.08 m Height of object in road 0.6 m Driver perception and reaction time 2.5 seconds (sec) Driver brake response deceleration at 3.4 m/s2

The driver eye height is exceeded by the majority of vehicles in the common fleet. The height of the object is based on the taillights of a vehicle. The background on this model is described in (NCHRP 400, 1997).

SSD design values are calculated as the sum of the two distances representing driver brake reaction distance and vehicle braking distance. Brake perception and reaction distance is the distance traveled by the vehicle from the instant the driver detects the object on the roadway and then applies the brake. Braking distance is the distance traveled by the vehicle from the instant the brake is applied to where the vehicle comes to complete stop under the assumed deceleration rate. The following equation is used to calculate SSD.

SSD = Brake Reaction Distance + Braking Distance

SSD = . + .

where SSD = stopping sight distance, m, V = design speed, kph, t = brake perception and reaction time, 2.5 seconds a = deceleration rate, m/s2, 3.4 m/s2

2.2.2 Stopping Sight Distance Design Values Table 2.1 lists the recommended computed stopping sight distance design values. These apply on grades of less than 3 percent. The values shown are minimum values. Table 2.1 provides SSD values for a full range of potential selected design speeds in even 10 kph increments. By policy, design speeds are limited to specific values for each functional classification as shown in Table 1.1. However, designers may select a design


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speed other than specified in Table 1.1 for a given functional classification through departure process specified in Part 25, Departures from Standards Process.

Table 2.1 Stopping Sight Distance for Level Roadways with Grades less than 3 Percent

Design Speed (kph) Brake Reaction

Distance (m) Braking Distance

(m)

Stopping Sight Distance (m)

Calculated (m) Design (m)

20 13.9 4.6 18.5 20

30 20.9 10.3 31.2 35

40 27.8 18.4 46.2 50

50 34.8 28.7 63.5 65

60 41.7 41.3 83.0 85

70 48.7 56.2 104.9 105

80 55.6 73.4 129.0 130

90 62.6 92.9 155.5 160

100 69.5 114.7 184.2 185

110 76.5 138.8 215.3 220

120 83.4 165.2 248.6 250

130 90.4 193.8 284.2 285

140 97.3 224.8 322.1 325

Note: Shaded values are for design speeds selected only through the Departure process per Part 25, Departures from Standards Process. Source: AASHTO, 2011a.

In general, terrain in Qatar is flat and the SSD design values in Table 2.1 meet the requirements for level roads with grades less than 3 percent. For grades steeper than 3 percent, the SSD design values could be calculated using the formula shown below (AASHTO, 2011a). SSD = 0.278 Vt + 0.039

. where SSD = stopping sight distance, m V = design speed, kph t = brake reaction time, 2.5 seconds a = deceleration rate, m/s2, 3.4 m/s G = grade, m/m

Roadway geometry that limits the available sight distance includes both vertical alignment and combinations of horizontal alignment and roadside obstructions. Design parameters for SSD are thus required for all three dimensions. On a tangent roadway, drivers line of sight may be limited by the vertical alignment of the roadway surface, specifically at crest vertical curves. On horizontal curves, the line of sight may be limited by obstructions outside the traveled way, such as bridge piers, retaining walls, bridge approach fill slopes, concrete barriers, guardrails, buildings, back slopes in cut areas,


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etc. Providing SSD thus requires analysis and design in both horizontal and vertical planes.

2.2.3 Horizontal Restrictions to Stopping Sight Distance Horizontal sight lines are based on an assumed location of the drivers eye within the roadway or lane. The sight line on a horizontal curve is defined by a chord, which assumes the eye location is centered on the travel lane, and the object is a point centered in the road along the curve. The SSD is measured along the centerline of the lane, as shown in Figure 2.1.

To provide for the SSD as measured along the center of the lane, the sight line chord must not be obstructed by a feature outside the traveled way. The design process involved the calculation of what is referred to as the horizontal offset (HO), which is the radial dimension from the center of the lane to the limiting sight obstructing feature. Design for the horizontal offset to the obstruction is calculated using the following formula. HO = R .

Or the SSD can be calculated for a given horizontal offset using the formula S = .

where S = stopping sight distance, m HO = horizontal offset measured from the centerline of inside lane, m R = radius to centerline of inside lane


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Source: AASHTO, 2011a. Figure 2.1 Horizontal Stopping Sight Distance

HSSD should be provided for the entire length of the curve and both directions of travel. The most critical case for a two-lane road will involve the travel lane on the inside of the curve.

The formula to calculate HO produces exact results when the length of the curve is greater than the required SSD, in which case both the driver eye location and the point obstruction are within the limits of the horizontal curve. If the required SSD is greater than the length of the curve, either the vehicle or the obstruction will be outside the limits of the horizontal curve. In these cases, the values for HO produced by the formula

= 1 28.65

Where: HO = Offset to the sight obstruction measured from centerline of inside lane, m S = Stopping sight distance along the curve, m R = Radius of the centerline of inside lane, m

= 28.65 1


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are approximate and may be slightly greater than required and in many cases, it may not be significant. In these instances and when horizontal alignment consists of combination of spirals, curves and tangents, HO can determined graphically.

An obstruction to SSD may be a point location (as shown in Figure 2.1) or a continuous obstruction such as a retaining wall concentric with the curve.

2.2.4 Vertical Restrictions to Stopping Sight Distance Vertical restrictions to SSD on crest and sag vertical curves. The most common restriction is a crest vertical curve as illustrated in Figure 2.2. The SSD design parameters for eye height and object height noted above define the sight line which the crest curve should provide.

Chapter 4 provides details on design lengths for vertical curvature to provide the necessary sight lines for SSD.

Figure 2.2 Stopping Sight Distance at Crest of Vertical Curve

Vertical restriction, as illustrated in Figure 2.3, on sag vertical curves depends on the ability of the driver to see the roadway surface from the beams of headlights at nighttime with the following assumptions:

Height of the head light: 0.6 m Height of the object: 0 m 1 degree upward divergence of headlight beam


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.

Figure 2.3 Stopping Sight Distance at Sag Vertical Curve

2.3 Passing Sight Distance Two-lane rural highways may be designed to enable the ability of drivers to pass slower vehicles using the opposing traffic lane. The concept of Passing Sight Distance (PSD) expresses the length of sight line required for a driver to affect a passing maneuver without coming into conflict with oncoming traffic. It applies only to two-lane undivided roadways where the fast moving vehicles overtake slow moving vehicles. PSD is the distance required for a driver to observe the oncoming vehicle traveling in the opposing direction and to complete the passing maneuver safely without conflict with the opposing vehicle.

Derivation of design values for PSD is based on a three-step model shown in Figure 2.4. The model assumes the passing vehicle (passenger car) accelerates to design speed, and the speed of the vehicle being passed (passenger car) is traveling at the design speed, while the approaching vehicle (passenger car) traveling in the opposing direction at design speed.

Figure 2.4 Passing Maneuver


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D1 shows the distance required to complete the passing maneuver by vehicle A. D2 shows the distance traveled by vehicle B in the opposing lane as vehicle A overtakes vehicle C. D3 shows the distance required between the opposing vehicles at the end of the passing maneuver. PSD is sum of the distances D1, D2, and D3. Distances are calculated using the formulas below. Source: Design Manual for Roads and Bridges (2002). D1 = 0.85 t V D2 = t V and D3 = D2/5 PSD = D1+D2+D3 = 2.05 t V where PSD = passing sight distance, m t = time to complete the passing maneuver, 10 sec V = design speed, m/sec

Table 2.2 lists the passing sight distance design values. Minimum values are shown. Where practical consider using higher values as the basis of design. Table 2.2 provides PSD values for a full range of potential selected design speeds in even 10 kph increments. By policy, design speeds are limited to specific values for each functional classification as shown in Table 1.1. However, designers may, select a design speed other than specified in Table 1.1 for a given functional classification through departure process specified in Part 25, Departures from Standards Process.

Table 2.2 Passing Sight Distance for Two-Lane Roadways

Design Speed (kph) Passing Sight Distance (m)

20 120

30 180

40 250

50 290

60 345

70 410

80 460

90 520

100 580

110 630

120 690

130 *

140 *

Note: Shaded values are for design speeds selected only through the Departure process per Part 25, Departures from Standards Process. * Not recommended for facilities with design speeds greater than 120 kph Source: Department of Transport, 2002


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The provision for PSD is not a design requirement; rather, its presence influences the capacity of the road. The Transportation Research Board Highway Capacity Manual (2010) describes methods for determining the capacity of two-lane rural highways. One of the parameters is the percentage of highway in which passing can occur. This would be the length of a roads alignment in which PSD is available divided by its total length.

PSD is measured using both an eye height and object height of 1.08 m. For design of crest and sag vertical curves using PSD, refer to Chapter 4. PSD should be checked in both horizontal and vertical plane. Procedures provided in Chapter 2, Sections 2.2.1 and 2.2.2 can be used to check the available sight distance by substituting PSD for SSD and using object height of 1.08 m.

2.4 Decision Sight Distance The minimum sight distance design requirement is provision for SSD. The SSD model is simple and expresses one specific driver action, which is braking in response to an object in the road.

The operating environment presents many other challenges to human drivers that are more complex, require more time, and involve different maneuvers or actions. Such maneuvers involve decision-making by the driver. The concept of decision sight distance (DSD) expresses the sight line to be provided a driver in advance of roadway conditions that require decision-making and then other maneuvers. Lengths of DSD are much longer than SSD given 1) decision-making takes longer time and 2) driver resultant maneuvers are different.

The decision sight distance (DSD) provides the additional length needed by the drivers to reduce the likelihood for error in perceiving the necessary information, making a decision, and executing the maneuver. Providing DSD is not a requirement, but consideration should be given in providing DSD at certain critical locations along the roadway. The following are examples of conditions for which designers should consider providing DSD in advance of the condition:

Exit and entrance ramps at the interchanges Left-hand exits on freeways or expressways High-speed roadway diverge and merge areas Change in cross section of the roadway, as in lane drops At-grade railroad crossings Signalized intersection on the downstream end of a crest vertical curves

The derivation of DSD includes two basic types of maneuvers: stop and speed, path, or direction change. The derivation also considers the context of the road (rural, suburban, and urban) which reflects driver expectations. The calculation of DSD depends on the design speed, type of roadway urban or rural and the type of avoidance maneuver needed to negotiate. QHDM adopts AASHTOs definitions for the five avoidance maneuvers:


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Avoidance Maneuver A: Stop on rural road Avoidance Maneuver B: Stop on urban road Avoidance Maneuver C: Speed, path, or direction change on rural road Avoidance Maneuver D: Speed, path, or direction change on suburban road Avoidance Maneuver E: Speed, path, or direction change on urban road

Table 2.3 provides DSD values. Where practical, consider using higher values as the basis of design. Table 2.3 provides DSD values for a full range of potential selected design speeds in even 10 kph increments. By policy, design speeds are limited to specific values for each functional classification as shown in Table 1.1, Chapter 1. However, designers may select a design speed other than specified in Table 1.1 for a given functional classification through departure process specified in Part 25, Departures from Standards Process.

The DSD values in Table 2.3 are determined using the following equations. For avoidance maneuvers A and B the equation is: DSD = . + .

For avoidance maneuvers C, D, and E the equation is DSD = . where DSD = decision stopping sight distance, m V = design speed, kph a = deceleration rate, m/s2, 3.4 m/s2 t = pre-maneuver time, seconds and varies with the avoidance maneuver t = 3 seconds for avoidance maneuver A t = 9.1 seconds for avoidance maneuver B t = varies between 10.2 and 11.2 seconds for avoidance maneuver C t = varies between 12.1 and 12.9 seconds for avoidance maneuver D t = varies between 14.1 and 14.5 seconds for avoidance maneuver E

The longer distances associated with urban conditions reflects the more complex, visually cluttered urban environment.

The application of DSD is encouraged. As it is not a specific requirement, choosing not to provide DSD does not require a Departure.


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Table 2.3 Decision Sight Distance

Design Speed (kph)

Decision Sight Distance (m) for Avoidance Maneuver

A B C D E

50 70 155 145 170 195 60 95 195 170 205 235 70 115 235 200 235 275 80 140 280 230 270 315 90 170 325 270 315 360

100 200 370 315 355 400 110 235 420 330 380 430 120 265 470 360 415 470 130 305 525 390 450 510 140 345 580 420 490 550


2.5 Intersection Sight Distance Drivers approaching and proceeding through intersections require sufficient sight lines to appropriately complete their maneuvers. Intersection sight distance (ISD) is a critical design element of intersection design. ISD is the distance required for a driver approaching an intersection to see the traffic on the intersecting roadway in order to safely cross or make a left or right turn on to the intersecting roadway. When two roadways intersect, numerous traffic movements occur that can create more vehicular conflicts. Providing adequate ISD at the intersection reduces the likelihood of such conflicts.

The driver approaching the intersection should have a clear view of the entire intersection and along the intersecting roadway to make the intended maneuver safely. ISD is determined by using the same principles as SSD, but it incorporates an additional element; driver behavior at the intersection.

Sight lines for ISD involve varying driver positions along one road, and the object being avoided another vehicle on the crossing road. These lines define what are referred to as sight triangles.

Sight triangles are employed in establishing the ISD. In general, for a typical intersection with four approaches, there are four quadrants. The sight line establishes a triangular wedge in each quadrant between the intersection roadways, called sight triangles. The sight triangles should be clear of obstructions that may block a drivers view of conflicting vehicles on the intersecting roadway. The triangle legs shown in Figure 2.5 should be long enough that drivers approaching the intersection from the two intersecting roadways can see each other to avoid collision and make the intended maneuver safely.


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In Qatar most intersections are priority intersections (three-intersections) with yield control on the minor approach. Four-legged intersections are rare and designed infrequently. For ISD sight triangle illustration purposes and to capture all possible maneuvers, through, left and right from the minor street, a four-legged intersection is chosen as an example.

The sight line defining ISD in both horizontal and vertical plane is based on an eye height and object height of 1.08 m.

The dimensions of the sight triangles depend on the design speed of the major roadway, type of intersection control (uncontrolled, yield control, stop control or signal control). QHDM adopts the AASHTO procedures to determine ISD for the following types of traffic control:

Case A: Intersections with no control

Case B: Intersections with stop control on the minor road

B1: Left turn from the minor road

B2: Right turn from the minor road

B3: Crossing maneuver from the minor road

Case C: Intersection with yield control on the minor road

C1: Crossing maneuver from the minor road

C2: Left or right turn from the minor road

Case D: Intersections with traffic signal control

Case E: Intersection with all-way stop

Case F: Left turns from the major road

2.5.1 Case A: Intersections with No Control Case A may be applicable for low volume and low speed intersecting roadways that are not controlled by yield signs, stop signs, or traffic signals. Figure 2.5 illustrates the sight triangles on the major road and the minor road. Distances a1 and a2 are from the major road to the decision point, location of drivers eye, along the minor road. The decision point is the point where the driver on the minor road has a clear view of the intersection and the vehicles approaching the intersection from the major road. The decision point is represented by the vertex of the sight triangle as shown on Figure 2.5 on the minor road. At the decision point the driver approaching the intersection from the minor road makes the decision whether to slow down and stop or make the intended maneuver, left, right or through without stopping. Distance b is the required sight distance along the major road. Distance a2 is equal to a1 plus the additional width as required. Case A should be used only if authorized by the Overseeing Organization.


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Table 2.4 shows values for a1 and b along the minor and major approaches. The values shown are minimum values. Where practical, consider using higher values as the basis of design. Table 2.4 provides ISD values for a range of potential selected design speeds in even 10 kph increments, from 20 kph to 80 kph. By policy design, speeds are limited to specific values for each functional classification as shown in Table 1.1, Chapter 1. Designers may, however, select a design speed other than specified in Table 1.1 for a given functional classification through departure process specified in Part 25, Departures from Standards Process.

Table 2.5 provides the factors for the approach grade adjustments.

Figure 2.5 Sight Triangles (Uncontrolled and Yield Controlled)

Table 2.4 Length of the Sight Triangle Legs, for Intersections with No Control

Design Speed (kph) Length of Legs a1 and b (m)

20 20

30 25

40 35

50 45

60 55

70 65

80 75

Notes: 1. For approach grades greater than 3 percent, multiply the sight distance values in this table by the appropriate adjustment factor from Table 2.5. 2. Shaded values are for Design Speeds selected only through the Departure Process per Part 25, Departures from Standards Process. Source: AASHTO, 2011a.


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Table 2.5 Adjustment Factors for Intersection Sight Distance Based on Approach Grade

Approach Grade (%)

Design Speed (kph)

20 30 40 50 60 70 80 90 100 110 120 130 140

-6 1.1 1.1 1.1 1.1 1.1 1.1 1.2 1.2 1.2 1.2 1.2 1.2 1.2

-5 1.0 1.0 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.2 1.2 1.2 1.2

-4 1.0 1.0 1.0 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1 1.1

-3 to +3 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0

+4 1.0 1.0 1.0 1.0 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

+5 1.0 1.0 1.0 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

+6 1.0 1.0 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

Source: AASHTO, 2011a

For an intersection to be considered compliant with the ISD values, the area within the sight triangle should be clear of obstructions. This enables each driver approaching the intersection to see the other, adjust speeds accordingly, and negotiate the intended maneuver without a conflict.

2.5.2 Case B: Intersections with Stop Control on Minor Road For intersection with stop control on minor roads, sight triangles should be checked for the following scenarios:

Case B1: Left turn from minor road Case B2: Right turn from minor road Case B3: Crossing the major road from minor road

2.5.2.1 Case B1: Left Turn from Minor Road

Figure 2.6 shows the sight triangles at stop controlled intersections. The decision point represents the position of the drivers eye on the minor road where the vehicle is stopped before carrying out the intended maneuver. The distance to the decision point from the edge of the major road traveled way should be at least 4.4 m; 5.4 m is desirable. Distances a1 and a2 are from the major road centerline of the lanes to the decision point (location of drivers eye) along the minor road as shown in Figure 2.6. Distance b is the required sight distance along the major road. Distance a2 is equal to a1 plus the additional width as required. Minimum length a1 is equal to 4.4 m plus the width of pavement from the edge of the major road traveled way to the centerline of the lane as shown in Figure 2.6 and desirable is equal to 5.4 m plus the width of pavement from the edge of the major road traveled way to the centerline of the lane


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Figure 2.6 Sight Triangles (Stop Controlled)

ISD required on the major road is calculated using the following equation (AASHTO, 2011a): b = ISD = 0.278x Vmajor tg where ISD = intersection sight distance Vmajor = design speed on the major road, kph tg = time gap for minor road vehicle to enter the major road, seconds; tg depends

on the design vehicle and also number of lanes on the major roadway and the values in Table 2.6.

Table 2.6 Time GapCase B1, Left Turn from Stop

Design Vehicle Time Gap, tg, at Design Speed of Major Road (seconds)

Passenger car 7.5

Single unit truck 9.5

Intermediate Semitrailer 11.5

Note: Time gaps are for stopped vehicle to turn left on to a two-lane highway with no median and with grades of 3 percent or less. The table values are adjusted as follows: For multilane highwaysFor left turns on to two-way highways with more than two lanes, add

0.5 second for passenger cars or 0.7 second for trucks for each additional lane, from the left, in excess of one, to be crossed by the turning vehicle.

For minor road approach gradesif the approach grade is an up grade that exceeds 3 percent, add 0.2 second for each percent grade for left turns.

Source: AASHTO, 2011a.


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Table 2.7 lists ISD values. The values shown are minimum values; where practical, consider using higher values as the basis of design. Table 2.7 provides ISD values for a full range of potential selected design speeds in even 10 kph increments. By policy, design speeds are limited to specific values for each functional classification as shown in Table 1.1. However, designers may select a design speed other than specified in Table 1.1 for a given functional classification through departure process specified in Part 25, Departures from Standards Process.

Table 2.7 Intersection Sight DistanceCase B1, Left Turn from Stop

Design Speed (kph) SSD (m) Intersection Sight Distance for Passenger Cars (m)

20 20 45

30 35 65

40 50 85

50 65 105

60 85 130

70 105 150

80 130 170

90 160 190

100 185 210

110 220 230

120 250 255

130 285 275

140 325 295


2.5.2.2 Case B2: Right Turn from Minor Road

ISD is required on the major road for the case in which a vehicle is turning right from a stop condition from the minor road. The ISD is calculated using the same equation as that used for Case B1, with tg values as given in Table 2.8. Table 2.9 shows the ISD values. The values shown are minimum values. Where practical, consider using higher values as the basis of design. Table 2.9 provides ISD values for a full range of potential selected design speeds in even 10 kph increments. By policy, design speeds are limited to specific values for each functional classification as shown in Table 1.1. However, designers may select a design speed other than specified in Table 1.1 for a given functional classification through departure process specified in Part 25, Departures from Standards Process.


VOLUME 1 PAGE 27

Table 2.8 Time GapCase B2, Right Turn from Stop and Case B3, Crossing Maneuver

Design Vehicle Time Gap, tg, at Design Speed of Major Road

(seconds)

Passenger car 6.5

Single unit truck 8.5

Interme

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