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Geometric Design

Elements of GD

• Cross section elements

• Sight distance considerations

• Horizontal alignment details

• Vertical alignment details

• Intersection elements

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Cross-Section Elements

• Travel lanes

• Shoulders

• Medians

• Roadside barriers

• Guardrails

• Side Slopes

• Curb and Gutter (in urban areas)

Design Control and Criteria

1. Functional classification

2. Projected traffic volumes and composition

3. Design speed

4. Design vehicle

5. Vehicle mix

6. Topography

7. Right-of-Way (ROW)

8. Costs and Available Funding

9. Driver performance factors

10. Social and environmental impacts

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1. Functional Classification

• Principal arterials

• Minor arterials

• Major collectors

• Minor collectors

• Local roads and streets

2. Projected traffic volume

• Determine critical number of vehicles that will use the designed facility.

• Traffic Elements:a. Average Annual Daily Traffic (AADT) or Average Daily Traffic (ADT)

b. k factor (represents the percentage of traffic occurring during the peak hour during an average weekday

c. Design Hourly Volume (DHV) (represents the 30th highest hourly volume) during a year

d. Directional Distribution (D)

e. Percentage of Trucks and heavy vehicles

f. Design Flow rate (V) – Peak 15-minute flow rate

• Relationships– DHV = ADT (k)

– V = DHV/PHF

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2. Projected traffic volume

• American Association of State Highway and Transport Officials (AASHTO) recommends:a. k value of 8 to 12 percent for urban facilities and 12

to 18 percent for rural facilities.

b. Directional distribution for rural roads is generally 55 to 80 percent with an average value of 6.Directional distribution for urban roads is generally assumed to be 50.

c. The compositon of heavy vehicles (T) in the traffic stream during the design hour typically varies from 5 to 10 percent but in some cases can be as high as 25 percent

3. Design Speed

• A selected speed to determine the various geometric features of a roadway

• Design Speeds range from 20 mph to 70 mph in increments of 10 mph.

• Design Speed depends on the functional classification of the highway (expected traffic volume), the topography of the area and the adjacent land use.

• Freeways are designed for 60 to 70 mph speeds. Design speeds are selected to achieve a desired level of operation and safety on a highway. With improvements in traffic control and vehicular technology, the design speeds are increasing with time.

• Rolling and Mountainous terrains have different design speeds. When the highway type or topography warrants a change in design speed, the speed is changed gradually.

• School, residential areas etc. typically have lower design speeds.

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4. Design Vehicle

• Design vehicles are selected to represent all

vehicles on the highway. The vehicle type

selected is typically the largest vehicle likely to

use the highway with considerable frequency.

• The weight, physical dimensions, and

operating characteristics of the design vehicle

will be used to establish the geometric

features of the highway

5. Vehicle Mix

• Vehicle mix may dictate special considerations

e.g. special bicycle lanes, HOV lanes, Bus

lanes, etc.

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6. Topography

• Alignment

– Rivers, built-up areas, mountains, etc.

• Gradient

– Permissible maximum 3%

7. Right-of-Way (ROW)

• Available ROW>=Required ROW

• Required ROW =

– Travel lanes +

– Shoulders +

– Medians +

– Roadside barriers +

– Guardrails +

– Side Slopes +

– Curb and Gutter (in urban areas)

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8. Costs and Available Funding

• Estimate construction cost and compare with

available funding

• IRR, NPV, B/C ratio, etc.

9. Driver performance factors

A. Sight Distance

• Length of road visible ahead of the driver of a vehicle. Sight distance at any point should be as long as possible but never less than the minimum stopping distances

1. Stopping sight distance

Stopping sight distance is the minimum distance required to stop a vehicle traveling near the design speed before it reaches as stationary object in the path

2. Passing sight distance

Passing sight distance is the minimum distance required by a vehicle traveling near the design speed for overtaking another vehicle

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Stopping Sight Distance

• Factors

– PIEV time

– Driver eye height

– Object height

– Vehicle operating speed

– Pavement coefficient of friction

– Deceleration rate

– Roadway grade

PIEV Time

• PIEV= Perception, Identification/Intellection, Emotion/Judgment, Volition or reaction– Perception: Seeing the object along with other objects

– Identification: Identification and understanding the stimuli

– Emotion/Judgment: Deciding the course of action (e.g. stop, overtake, move laterally, blow horn, etc.)

– Volition: Execution of the decision

• Typical PIEV values:– Urban 2.25-2.5 sec

– Rural 2.5 sec

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Factors affecting PIEV

• Complexity of the situation

• Gravity of the event

• Driver’s age and cognitive abilities

• Mental/psychological conditions

• Roadway setting

• Speed

Calculating Stopping Sight Distance

S= Sight distance available over crest of vertical curve

L= length of vertical curve

A= algebraic difference in grades (%)

h1=height of eye of average driver (typically 1.07m/3.5 ft)

h2= height of object

S<L: � = ��2

100(√2ℎ1+�2ℎ2) 2

S>L: � = 2� − 100(√2ℎ1+�2ℎ2) 2

�

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Calculating Passing Sight Distance

S= Sight distance available over crest of vertical curve

L= length of vertical curve

A= algebraic difference in grades (%)

h1=height of eye of average driver (typically 1.07m/3.5 ft)

h2= height of eye of average vehicle (typically = 1.30 m/ 4.25 ft)

S<L: � = ��2

100(√2ℎ1+�2ℎ2) 2

S>L: � = 2� − 100(√2ℎ1+�2ℎ2) 2

�

Problem

• Calculate the minimum stopping sight

distance if the sight distance available over

crest of vertical curve is 8ft, length of vertical

curve is 5ft, difference in grades is -5% and the

object height is 2ft.

• What is the minimum passing sight distance

for the same location?

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10. Safety

• Principles

– Design for all users

– Reduce conflicts

– Encourage appropriate speed and behavior by

design

• Speed humps, rumble strip, narrowing

– Avoid surprises and confusion

• Signs

– Create forgiving road

11. Social and environmental impacts

• Social impacts

– Relocation due to land-acquisition

– Disturbance during construction period

• Environmental impacts

– Particularly critical near schools, hospitals

• Noise

– Noise barriers, low-noise surfacing on roads, noise insulation

in homes

• Air quality

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RHD Design

Process

1. Define basic parameters

• Determine the design year traffic volume

(usually the traffic in the 10th year after

opening)

• The type of terrain

• The High Flood Level

…and other basic parameters.

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2. Select design type

3. Check if all elements fall within design

speed standard/ one step below

• Check if all the elements in the trial alignment conform to the design speed

standards for the road (or section as appropriate).

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3. Check if all elements fall within design

speed standard/ one step below

• Check site constraints

– If this is not possible consideration must be given to

either redesigning the problem element or installing

very prominent signing, safety barriers, etc.

– Greater care and consideration is needed before

relaxing standards on high flow / high speed roads

– Reduction in standards should only apply to stopping

distances and curvature. Carriageway and shoulder

widths should not be reduced because this could

seriously affect capacity and safety.

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4. Check consistency with approach

speed estimates

• Ensure that actual speeds do not differ by more than about 15km/h on successive sections of the road. – When estimating actual speeds take account of the

geometry as well as the speed characteristics of the preceding few kilometres – sometimes called the “speed environment”. Actual speeds on a 500m radius curve in a 80km/h speed environment may be close to 80km/h but speeds on the same radius curve in a 60km/h environment will be very much lower.

– Long straights can induce very high speeds that may be well in excess of the overall design speed for the road

– Design speeds of the first curves at the ends of such long straights should not be more than 10km/h below the estimated speeds on the straights.

5. Check the Economic Assessment

• The full economic assessment of the project will have been done at the feasibility study stage.

• During the design process any alignment options that could improve the economic returns will need to be investigated. Once the design is firmed up check that the project cost has not altered sufficiently to significantly affect the economic return.

• If the economics of the project are not good then the project will have to be reviewed.

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Problem

• Design a two lane two way straight highway

section using the following info:

– Projected volume: 1000 PCU/ peak hr

– Plain terrain with design speed =80kmph

– Available sight distances are 600m

Horizontal Alignment

• Combination of tangent and curved

components

– Straight

– Circular curve

– Transition curve

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Components of Circular Curve

• BC - point of curvature

• PI - point of intersection

• EC - point of tangency

• T - length of tangent (BC - PI)

• M - Middle ordinate

• L - length of curvature

• D - external angle (in degrees)

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Length of Curve

• L = R Δ / 57.3, R=Radius, Δ=deflection angle

– Note that for a given external angle the length of

curve is directly related to the radius

– In other words, the longer the curve, the larger

the radius of curvature

Minimum Radius

• Radius constrained by forces acting on the occupants of a vehicle negotiating a curve and the resulting comfort level of the occupants.

– function of the velocity (V), the allowable side friction (f) and the degree of superelevation (e)

– Relationship:

(0.01e+f)/(1-0.01ef) = V2/gR

f in fraction, e in %

• Minimum radius->max velocity, max superelevation and max side friction

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Maximum Superelevation

• The practical factors limiting superelevation are– Weather

– Low Speed

– High CG/Loose suspension of some cars

• Therefore, the emax is selected based on the climate, and the likelihood of slow moving traffic

• Recommended Values for emax– Absolute maximum value recommended (expect on gravel roads) is

12%

– Maximum value in areas with snow and ice is 8%

– Maximum value in areas where slow traffic is likely (urban areas) is from 4% to 6%

• No superelevation is recommended in urban areas where congestion is expected

Maximum Side Friction

• The allowable side friction is selected to be

comfortable and safe for all car occupants.

• Typical values: 0.17 at 30 kph, 0.10 at 110 kph

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Intersection/Junction Design

• Serve a special function in accommodating

travel in opposing or conflicting directions

• Intersections have a disproportionate effect

on the overall safety (over 41% of arterial

accidents) and capacity of highways

Major Design Objects

• Vehicle occupants safety

• Pedestrian safety and comfort

• Effective traffic flow

• Match with surrounding land use

• Environmental and aesthetic fit

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Horizontal Element

• Angle of Intersection

– The angle of intersection should be approximately

equal to 90 degrees. An angle of intersection of

greater than 60 degrees is considered acceptable

• Problems with acute angle include

– Larger pavement area

– Visibility problems (blind spot for trucks, difficult

to check for vehicles on cross-road)

– Longer exposure for crossing/veh and ped

Horizontal Element

• Intersections should not be on sharp horizontal curves

• Problems include

– Reduced sight distance

– Operational Difficulties (divided driver attention since driver is concentrating on tracking, less friction available for braking, adverse superelevation for turning)

– Construction and Design Difficulties (matching superelevation to cross-section of intersecting roadway)

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Profile

• The grade at an intersection should be less than 3% for both intersecting roadways

• Problems with steeper grades:– Reduce discharge rate - affect flow through intersection

– Greater sight distance needed at intersection (Since acceleration rates decrease)

– Greater vehicle control problems

– Construction and design challenge of matching cross-sections

• The design of the cross-section in the intersection is achieved by carrying the cross-section of the major road through the intersection

Radius of Turn

• The radius needed is a function of the design vehicle and the desired speed of turn and the context.

• Three different designs are used – simple curve

– simple curve with taper

– compound curve

• The compound curve provides a much closer fit to the vehicle path and therefore uses much less pavement area.

• Curve with taper is not as good a fit as compound curve but sometimes used because it is simpler to design.

• Compound curve maybe symmetrical or asymmetrical.

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Urban

• Generally smaller radii than for corresponding rural roads due to

– Lower speeds

– Space limitations

– Additional available space where there are parking lanes

– Effect of large radius on pedestrian

• Most cities use from 1.5 to 9 meters (most common, 3 to 4.5 meters).

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Intersection Sight Distance

• One of the most important design feature at the intersection is the amount of sight distance

• There should be enough sight distance for vehicles approaching the intersection and enough sight distance for vehicles already in the intersection

• AASHTO uses the concept of sight triangles of assessing the sight distance requirements in the vicinity of an intersection.

• The sight triangle maybe either an

– approach sight triangle

– departure sight triangle

Approach Sight Traingle

• The approach sight triangle allows vehicle approaching the intersection sufficient time to see the vehicle on the intersecting roadway and to take appropriate action.

• The length of the legs of the triangle is a function of the expected speed on the road in question

• The approach sight triangle is required where the intersection is has no control or a yield control on the minor road. It is not needed where the intersection is controlled by signs or signals.

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Departure Sight Traingle

• Provide sufficient distances for the driver on the

minor road to depart from a stopped position

and to enter or cross the major roadway

• The departure sight triangle is needed at each

location where departure is controlled by stop,

yield or signal control (for example, if the road is

a four-way stop, departure sight triangle is

checked for each quadrangle of the intersection)

Types of At-grade Intersections

• Cross

• T

• Roundabout/ Rotary/ Traffic Circle

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Interchanges

• Grade separated intersections

• Advantages?

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Interchanges

• Grade separated intersections

• Advantages

– Control Access road

– Bottleneck location

– Hazardous location

– Natural Topography (Fall Creek)

– Bikeway/Pedestrian way

– Access to isolated area

Types

• Diamond

• Cloverleaf

• Trumpet

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Diamond

• Simplest, most common and least costly type of interchange

• Generally used to connect freeway to a local road

• Off ramp from the freeway terminates with an at-grade intersection at the minor road (not applicable if minor road is limited access)

• Inexpensive - little ROW required and only one, simple structure

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Cloverleaf

• Collector-distributor road can be used to

alleviate weaving problem

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Trumpet