roadway cross section

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ROADWAY CROSS SECTION | TYPICAL HIGHWAY CROSS SECTION

HIGHWAY CROSS SECTION ELEMENTS1. LANE WIDTH OR HIGHWAY TRAVEL WAY:The width of the surfaced road and number of lanes should be adequate to accommodate the type and volume of anticipated traffic and assumed design speed of vehicles. Roads presently in use have traditionally been separated into generalized categories that include two-lanes, three-lanes, multilane undivided, multilane divided, and limited access highways.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 shoulders on either side along primary routes.

Three lane roads were built in previous years. The three lane road does appear to have a high accident rate, and construction of these types of roads is no longer recommended except in special casesOn four lane highways traffic flows in opposite directions on each pair of lanes, and passing is accomplished within the lanes of forward movement and not in the lanes of opposing traffic. In order to provide positive protection against conflict of opposing traffic, highways are frequently divided by a median strip. On such highways, lane widths should be minimum of 3.6 m, with 4 lanes provided where many large trucks combinations are anticipated.

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 1.2m to 18m.Where narrow medians must be used, many agencies install median barriers to physically separate opposing flows of traffic and minimize the potential for head on collisions. Medians of 4.2 m to 4.8 m have been used and are sufficient to provide most of the separate advantages for opposing traffic while permitting the inclusion of a median lane at crossroads for right turn movements, however , medians 4.8 m to 18 m wide and greater are now recommended when surrounding conditions permit.Many agencies design the width of medians so that additional travel lanes can be added in the future, if required, within the limit of the median. For rural divided highways a median width of 13.4 m is considered desirable to provide one 3.6 m travel lane in each direction.

ADVANTAGES of Median of a roadAre given as;1. The chance of freak accidents which might produce head on collision over a narrow median is reduced.2. Headlights glare from opposing traffic is less troublesome.3. 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.

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, curb or slope.shoulders should be wide enough to permit and encourage vehicles to leave the pavement when stopping. The greater the traffic volume, the greater is the likelihood of the shoulders being put to emergency use. Shoulders also provide lateral support for pavement subbase, base and surface courses. Shoulder widths are usually determined by the traffic volume and the percent of heavy vehicles. 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.widths greater than 3.0 m [10 ft] may encourage unauthorized use of the shoulder as a travel lane. The slope of the shoulder should be greater than that of the pavement.

DIMENSIONS:1. 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.2. Inside shoulder are often not as wide (often 4)3. In section with guardrails or other vertical elements, an additional 2 of shoulder widening should be provided

4. SLOPES in Roadway Cross Section:The graded area immediately adjacent to the graded roadway shoulder is called side slopeHighway Cross Section - Types of Side slopes:BACKSLOPE: - Slopes back to natural topography are known as back slope.Backslopes steeper than 3:1 may be difficult to maintain and need to be evaluated with regard to slope stability.FORESLOPE: - Slopes down to ditch are called foreslope.AASHTO states that foreslopes steeper than 3:1 (33%) are recommended only where conditions do not permit the use of flatter slopes.5. DRAINAGE DITCHES/CHANNELSDrainage channels perform the vital function of collecting and conveying surface water from the highway right-of-way. Drainage channels, therefore, should have adequate capacity for the design runoff, provide for unusual storm water with minimum damage to the highway, and be located and shaped to provide a safe transition from the roadway to the backslope. The depth of channel should be sufficient to remove surface water without saturation of the subgrade. upon the soil characteristics.Top of Form