Vertical Alignment

Terrain Type Level TerrainRolling TerrainMountainous Terrain

Grades Passenger cars can operate on a 3% upgrade under un-congested conditions has little effect on their performance.

Trucks and recreational vehicles are heavily affected by upgrade and downgrade highways.

Speed-distance curve for acceleration and deceleration for heavy duty trucks of 120 kg/kW are shown in Exhibits 3-59 and 3-60. Use the charts to estimate V after the vehicle travels 350 m on the 6% upgrade. The entering speed is 60 km/h.

Vertical Alignment

Grades Maximum Grade 5 % - 110 km/h7-12% - 50 km/h

Minimum G 0% for uncurbed and proper cross slope0.3% for high-type pavement properly

sloped and supported by firm subgrade0.5% for any others

Critical Lengths of Grade for Design

It is the maximum length of a designated upgrade on which a loaded truck can operate without an unreasonable reduction in speed.

Vertical Alignment

Critical Lengths of Grade for Design

Assumptions: 1) A loaded truck with 120 kg/KW is the representative truck2) Speed at entrance to critical length of grade is the average running

speed.3) Minimum speed on the grade below in which interference to the

following vehicles is considered unreasonable.

A common basis for determining critical length of grade is based on a 15 km/h reduction in speed of trucks below the average running speed of traffic.

Exhibits 3-63 and 3064 should be employed to determine the critical length of grade. Note that the curves in the exhibits are developed based on V = 110 km/h.

Vertical Alignment

Climbing Lanes

Climbing lanes for two-lane highways is not considered as a three-lane highway, but a two-lane highway with an added lane for vehicles moving slowly uphill.

Criteria:

Upgrade traffic flow rate in excess of 200 veh/housUpgrade truck flow rate in excess of 20 veh/hourOne of the following conditions exists

1) A 15 km/h or greater speed reduction is expected for a typical heavy truck

2) LOS E or F exists on the grade3) A reduction of two or more LOS is experienced when

moving from the approach segment to the grade

Vertical AlignmentMethods for Increasing Passing Opportunities on Two-Lane Roads

The methods are passing lanes, turnouts, shoulder driving, and shoulder use sections.

Passing Lanes: An added lane can be provided in one or both directions of travel to improve traffic operations in sections of lower capacity to at least the same quality of service as adjacent road sections. Procedures are provided on Page 255.

Turnouts: A turnout is a widened, unobstructed shoulder area that allows slow-moving vehicle to pull out of the through lane to give passing opportunities to follow vehicles. 60 m < d < 185 m See Exhibit 3-68.

Shoulder Driving A long-custom has been established for slow-moving vehicles to move to

the shoulder when another vehicle approaches from the rear and return to the traveled way after the following vehicle has passed.

Shoulder Use limited use

Sections

Vertical AlignmentEmergency Escape Ramps

When long, descending grades exist or where topographic and location controls require such grades on new alignment, the design and construction of an emergency escape ramp at an appropriate location is desirable to provide a location for out-of-control vehicles, particular trucks to slow and stop away from the main traffic stream.

Location: downgrade with damaged guardrail, gouged pavement surface, and spilled oil

Types: gravity, sandpile, and arrester bed

Example: An emergency escape ramp is design to have an upgrade of 10%. The arrester bed is to be constructed with loose gravel for entering speed of 140 km/h, L =?

Vertical Alignment

Vertical Curves

There are two types of vertical curves: crest and sag curves

All vertical curves are parabolic curves.

Drainage of curbed roadways on sag vertical curves needs careful profile design to retain a grade of not less than 0.5%.

Crest curves: Lmin = AS2/(100((2h1)0.5+(2h2)0.5) S < L

Lmin = 2S – 200 (h10.5 +h2

0.5)2/AS > L

Explain how to derive the formulas

Vertical Alignment

Vertical Curves

Crest Curves

Design Controls – SSD

The Lmin for different A to provide the min SSD for each design speed is shown in Exhibit 3-75 on Page 273.

Use examples to highlight how to use the Exhibit 3-75.

Design Controls – PSD

See Formulas on Page 275.

It is impossible to design crest curves to provide for passing sight distance

Vertical Alignment

Vertical Curves

Sag Vertical Curves

Four different criteria are used to establish minimum length of sag vertical curves: headlight sight distance, passenger comfort, drainage control, and general appearance.

Headlight Sight Distance: See Formula on Page 277.

Passenger Comfort: L = AV2/395 , about Lcomfort = 0.5Lheadlight

Drainage Control: Affect the Type III curve design with curb used

A minimum grade of 0.3 % should be provided within 15 m of the level point.

Vertical Alignment

Vertical Curves

Sag Vertical Curves

Drainage Control: K = 51 per percent change in gradeShow how to use the drainage line for getting Lmax, drainage control

L min < L max, drainage control V 100 km/h

General Appearance: Lmin = 30 A

Sight Distance at Undercrossings Structures block the view and sight

distance should be examined using the formula on Page 281.

Vertical Alignment

General Controls for Vertical Alignment

1) A smooth gradeline with grade changes should be sought for in preference to a line with numerous breaks and short lengths of grades

2) The roller-coaster or the hidden-dip design should be avoided.

3) Undulating gradelines should be evaluated before they are used.

4) A broken-back gradeline should be avoided.

5) On long grades, steep grades at the bottom and the flat grades at the top.

6) Sag vertical curves should be avoided in cuts unless adequate drainage can be provided.

Vertical Alignment

Combinations of Horizontal and Vertical Alignment

1) Horizontal and vertical alignments should not be designed independently, but coordinated each other.

2) Curvatures and grades should be in proper balance. Poor design with flat curvature or tangent alignment at expense of steep or long grades.

3) Vertical curvatures superimposed on horizontal curvature result in a pleasing facility.

4) Sharp horizontal curvature should not introduced on the top of a crest curve or the bottom of a sag curve.

5) Both H and V curves should be as flat as possible at intersections.

Vertical Alignment

Alignment Coordination in Design

1) 3-D design is a good future in considering V and H curves jointly.

2) Alignment coordination should begin at the preliminary design. 3-D visualization could help designers view the joint effects of V and H curves.

3) Adjustment should be made to obtain the desirable coordination.

4) See various exhibits on the combination of H and V curves.

Vertical Alignment

Other Elements in Geometric Design

Drainage Drainage facilities include bridges, culverts, channels, curbs, gutters, and others. Design of these facilities

need to consider the hydraulic capacities and locations.

AASHTO Highway Drainage Guidelines should be used.

Erosion Control And Landscape Development Erosion prevention is critical to highway design.

Normally it is considered in drainage design.

landscape development needs to consider the preservation of existing vegetation, transplanting of existing vegetation, and planting of new vegetation, selective clearing and thinning, and regeneration natural plant species and material

Vertical Alignment

Other Elements in Geometric Design

Rest Areas, Information CentersScenic Overlooks A safety rest area is a roadside area with parking facilities

separated from the roadway. An information center is a staffed or un-staffed facility at a rest area. A scenic overlook is a roadside area provided for users to park for scene viewing.

Lighting Lighting may improve the safety of a highway or street and the ease and comfort of operation.

Utilities Full consideration should be given to utilities including sanitary sewers, water supply lines, oil, gas, and petroleum product pipelines, overhead and underground power and communications lines, cable TV , wireless communications

towers, drainage lines, and heating mains.

Vertical Alignment

Other Elements in Geometric Design

Traffic ControlDevices Signs and marking are two directly related to the design

Check MUTCD for placement of signs and markings.

Traffic Signals in the design of intersections and urban streets

Noise Barriers Placement design of noise barriers in roadway.

FensingWork Zone These two elements should be considered in geometric design

also.