2.0 Cross Section Design

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TR 320

Geometric Design of Highways

(2 units)

Lecture 2:

Cross Section Design

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Objective of this Lecture

Aim of the lecture: To introduce road cross section elements,

their functions and design considerations

The learner should be able to: To sketch and label a typical road cross

section, describe the function of eachelement and list the design considerations

Use the knowledge to completeassignment no. 1 successfully and tocriticise designs

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TR 320 - Cross Section Elements

Contents

Pavementtype, cross fall

Lane width

Pavement widening at curves

Number of lanes

Shoulders

The roadside

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Pavement

Part of the roadway used as a travelledway

May consist of:

Earth (max. permeability, max. slope)

Gravel base-course/surface

Gravel base (etc) with surface dressing

Asphalt Concrete (AC) surface OPCC base/surface (min. permeability, min.

slope)

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Pavement type isselected on the basis of Traffic Volume

Weather/Climate

Soil Type Availability of Construction Materials and

Total Transport Costs

*Environmental considerationsdepletionof gravel is increasing becoming

important!

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Cross Slope

design is controlled by traffic operations

and drainage considerations

Big slope is better for quick drainage but

camber with more than 2.5% slope

creates side-sway problem if speed is

more than 80 km/hr

Parabolic section is better but difficult to

construct

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Cross Slope .

Critical consideration on curved sections

super elevation often provided for paved

roads where high speed (60 km/hr or

more) is expected (= outside urban areas)

Different pavement types require different

slopes due to differences in permeability of

the pavement materials

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The surface of the pavement must

have enough Skid Residence

Important to safety (Minimize skidding

accidents)

Critical for wet pavements:Ruttingwater accumulation

Polishingreduces micro texture

Bleedingcovers micro texture

Dirtycause pavement to loss skid

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Skid Residence

Alignment and pavement texture must

be designed to produce high initial

skid resistance Reduced probability of polishing

Cross slope with 2.5% slope or more

reduces probability of hydroplaning Curve radius, super-elevation and

max grades may be selected to

minimize chances of polishing 9


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Lane Widths - Capacity

Controls safety and comfort (LoS) to

a great extent

3.0 m to 3.9m lane widths used inUSA: 12ft = 3.64m is considered

IDEAL lane width

In Tz 3 m to 3.25 m is common ontrunk roads

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Lane Widths - Capacity

3.6 m (ideal) considered better than

3.0 because of reduced maintenance

costs and better operations for trafficlevels > 400 vpd

Capacity of a highway is affected

greatly by the lane width: if 3.64 m =100% (3,200 vph) then 3.0 m = 77%

(2,464 vph)11


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Pavement Widening

Vehicles occupy larger widths on

curves than on tangents

Amount of increase in occupationof the roadway depends on

- Curve radius- Vehicles dimensions

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Pavement Widening

Curves traced by rear wheels are

called swept curves

Extra widening can be ignored forpassenger cars on radii used on

highways.

See AASHTO for the estimation

of amount of widening needed.13


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Need for Pavement Widening depends on:

The design vehicle - HGV

Frequency of meeting on

curves (traffic demand)

Curve radius.

Speed (design or operating

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What happens if curve widening is

not provided?

Drivers will need to

concentrate more on curves

discomfort, fatigue

Possible reduction in operating

speed (on curves)

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Widening Attainment on Curves

How is widening applied?

Gradually - on the inside lane

Usually introduced on transition

length or

Along Super-elevation Runoff

Marked centre line on widenedcurve showed be placed (midway)

between the pavement edges.17


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How many lanes?

Number of lanes should be enough toaccommodate design volumes for the selectedlevel of service.

In the HCM approach several influencingparameters are considered in the selection ofnumber of lanes.

The HCM approach is considered the best

approach in terms of assessing traffic quality onthe basis of the principle of level of service. Notcovered in this course.

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Shoulders

Definition: a shoulder is the portion of theroadway continuous with the travelled way

whose function is to provide lateral support

to the base and surface course,

accommodates stopped vehicles, for

emergency use and sometimes for use by

NMT (Non Motorised Traffic)

Width varies from 0.6 m to 3.6 mShoulders may be paved, gravel or

earth/grass19


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Shoulders: Desirable features (for

drainage)

Should be flash with pavement

surface

Sloped and drain away from thecarriageway

Slope from 26% paved > 4% - 6%

gravel and for Grass up to 8% Cross slope break at super-elevated

sections should be a max of 8%20


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Benefits of shoulders

Space is provided for disabled vehiclesand for motorists to stop when consultingroad maps/directions

Space for evasive manoeuvers Sense of openness provided reduce driver

stress, increase SD in cuts (safety)

Sometimes may improve aesthetics Encourage uniform speed and therefore

capacity is increased

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Benefits of shoulders ..

Provides space for maintenance activities:storage of materials, temporary travelledway

Provides lateral clearance for signs andguardrails (Road furniture)

Allows storm water to be discharged

further from the travelled roadwayreduces seepage into the pavement baseand therefore slows down deterioration

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Structural support is given to the

pavement

Space provided for pedestrian and bicycle

use, detouring of vehicles during re-

construction, bus stops,

Usually kerbs are used in urban areas.

Parking lanes may serve stalled vehicles

otherwise traffic is disturbed

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Bus and other turnouts

Provide efficient and safe removal of busfrom the travelled way e.g. for arterialroads provision of deceleration lanes is

desirable Provide standing space to accommodate

expected number of vehicles: 15 m foreach bus, 3 m wide

Provide convenient exit from the busturnout

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The roadside: Horizontal Clearance

of Obstructions

Clear zonesprovides for

recovery of errant vehicles

Side ditch design (slopes andshape) to facilitate

Rec. e.g. AASHTOmin for lowspeed rural roads 10ft = 3.0 m for

urban kerbed roads 0.5 m25


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Kerbs

Drainage control

Pavement edge delineation

RoW reduction (critical in urbancentres)

Aesthetics

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Kerbs

Delineation of pedestrian

walkways

Reduced maintenance operations(pvmt edge).

Assistance in orderly roadsidedevelopment

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Other cross section issues

Medians

Roadside barriers on medians

Guardrails

Sidewalks

Side slopes Right of Way (RoW)

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Summary: Basis for cross section

(CS) design

Limited research findings

Practical experience and judgement

Economical and environmentalassumptions

Administrative requests

CS Design decisions are not wholly

explained on purely scientific basis

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Thank you.

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2.0 Cross Section Design