The typical cross section provided by AASHTO for a two lane highway is given below.
Also see Road Structure
Highway Cross Section Elements
1. LANE WIDTH OR HIGHWAY TRAVEL WAY:
In meeting oncoming vehicle or passing slower ones, the portion selected by a driver depends primarily on the paved surfaced width of the highway. The width of the surfaced road and the no of lanes should are adequate to accommodate the type and volume of traffic anticipated and the assumed design speed of vehicles.
As traffic density, vehicle speed and truck widths have increased, two lane highway have also increased in width from 16’ to the current recommended value of 24’ width 10; stabilized or paved shoulders on either side along primary routes.
Standard for the interstate system set lane width at 11’ for rural roads when traffic density is less than 200 vehicles per lane per hour. For other rural and all urban facilities widths are set to be 22’. For primary highway carrying over 200 vehicles, lane widths are 11’ per 12’ depending on design speed and no of commercial vehicles. For secondary roads desirable lane is 10’.
2. MEDIAN STRIP FOR DIVIDED HIGHWAY:-
In order to provide positive protection against a conflict with opposing traffic median strop are provided on divided highways. The width of these median strip varies from 4’ to 60’.
Where median strips are narrow separation, is merely provided by raised curbs and, where greater widths are available curbs may (or may not) be used. Often in rural areas the wide division aline serves the purpose and no raised barrier is employed.
ADVANTAGES of Median of a road
Are given as;
- The chance of freak accidents which might produce head on collision over a narrow median is reduced.
- Headlights glare from opposing traffic is less troublesome.
- At intersection, a wide median provides refuge from crossing traffic.
Due to high cost of land, narrow width is provided at normal sections and added widths where right turn or x-traffic are to be accommodate are used. Medians are often narrowed at grade separation in order to reduce the length or width of the structure. Median width of 60’ or wide are inadvisable at signalized intersection locations because extra time is required.
WIDTH - (INTERSTATE)
3. SHOULDERS in Highway Cross Section :
It is that portion of the roadway between the outer edge of the outer traffic lane and the inside edge of the ditch, gutter, curb or slope. Shoulders are provided for the safe operation and to allow the development of full traffic capacity. Shoulder provides a place for vehicle to park for changing tires.
- Outside shoulders are width of at least 10’ and preferably, 12’ that is clear of all obstructions is desirable for all heavily traveled and high speed highways.
- Inside shoulder are often not as wide (often 4’)
- Mountainous areas: due to extra cost, the width is kept less, the use of partial shoulder may be permitted (protrude 1’-4’ into adjacent lane) under these conditions, emergency parking pull outs are provided.
- In section with guardrails or other vertical elements, an additional 2’ of shoulder widening should be provided.
It is common to pave it inside from 18” to 3’ with bituminous material or at least to apply bituminous surface treatment. In some instances, full width is paved or treated. Turfed shoulders are provided in areas with sufficient rainfall. It is common practice to mark the line between roadway and shoulder as guide.
Slope of the shoulder should be greater than that of pavement shoulder with high type surface. Slope form 2 to 6%, gravel 4 to 6% and turf 6 to 8% to assure efficient drainage away from pavements.
- One argument for wide, continuous shoulder is that they add structural strength to the pavement.
- Outside shoulder increase horizontal sight distance on curve.
4. SIDE SLOPES in Roadway Cross Section:
The graded area immediately adjacent to the graded roadway shoulder is called side slope
Highway Cross Section - Types of Side slopes:
BACKSLOPE: - Slopes back to natural topography are known as back slope.
FARESLOPE: - Slopes down to ditch are called fare slope.
Slope depend upon soil characteristics and the geographic location of the highway.
FLAT SLOPES are preferred because it provide.
- Safe operation. 2. Decreased maintenance.
STEEP SLOPES erode badly, require high maintenance cost and slopes will be unsightly.
DIMENSIONS: - or width or requirement for different side slope conditions are;
- Recommended side slope in cut à not steeper than 2 to 1 except in solid rock.
- For fill à <10’ height, side slope are set at 4 to 1 or flatter where height of cut or fill doesn’t exceed 6’, 6 to 1 slide slop is recommended.
- Guard rail à where guard rails are provided slope may be as high as 1.5 to 1.
When cut or fill slope intersects the ground surface, slop should be rounded to blend with the natural ground surface.
It is often preferable to contain the embankment with a suitable retaining wall (because in rural steep country slopes does not usually catch natural ground and in urban areas there is narrow right of way.
CROSS-SLOPES OR CAMBER: - The slope provided to road surface in the traverse direction to drain off rainwater from road surface is called cross-slopes.
It is introduced in all tangent sections of the roadway except at curves where super elevation directs all water toward the inside.
Importance: - It is important
- To prevent entry of surface water into the sub grade soil through pavement.
- To drain off water quickly from road surface as it will make the road slippery and will not be sage for high speed vehicle.
SHAPE : - The different shapes of cross-slopes are given below;
- Straight line
- Combination of straight and parabolic line
RATE OF CAMBER : - The rate of caber depends upon’
- The type of pavement surface.
- Amount of rainfall.
For high pavement à after 1/8in per ft
For steep slopes should be avoided.
RECOMMENDED VALUES : - (IRC) the range is based upon rainfall;
Right of waysàRight of way area of the road acquired for road (carriages way + other necessities + future extension), along its alignment.
- Good practice is to acquire right pf way wide enough to accommodate ultimate development.
- Factors à depend upon.
àwidth of formation àheight of embankment
àside slopes àdrainage system
àsight distances consideration on horizontal curves
- Width : - According to AASHTO
For two lane highway secondary roads AADT 400 – 1000
Min = 66’ Desirable = 80’
Interstate highway system
From 150’ without frontage roads
From 250’ with frontage roads
Up to 200’ – 300’ for eight lane divided highway without frontage roads.
On high type two lane highway in rural area.
Min = 100’ Desirable = 120’
For urban areas acquisition be wide enough to be adequate for ultimate design (no specified width)