Medians/Traffic separators , kerbs, Road margins

Medians/Traffic separators , kerbs, Road margins , Shoulders , Guard rails, embankment slopes

1. DEPARTMENT OF CIVIL ENGINEERING Medians/Traffic separators Kerbs Road Margins Er . Ramprasad Kumawat M.Tech

2. CONTENT Medians/Traffic separators Kerbs Road Margins

3. Medians/Traffic separators In highways with divided carriageway, a median is provided between two sets of traffic lanes intended to divide the traffic moving in opposite directions. The main function of the median is to prevent head-on collision between vehicles moving in opposite directions on adjacent lanes. The median is also called or traffic separator.

4. The traffic separators used may be in the form of pavement markings, physical dividers or area separators. Pavement marking is the simplest of all these, but this will not rule out head-on collision. The mechanical separator may be suitably designed keeping in view safety considerations.

5. The medians/traffic separators may also serve the following functions: (a) to channelize traffic into streams at intersections (b) to shadow the crossing and turning traffic (c) to segregate slow traffic (d) to protect pedestrians

6. It is desirable to provide wide median/traffic separators of 8 to 14 m width so that it is possible carry out future widening of the carriageway towards the median. But the width should be decided in conformity with the availability of land and its cost . A minimum of 6 m is required to reduce head light glare due to vehicles moving in opposite directions on either side of the median at night. The glare can be reduced in narrower strips by planting shrubs.

7. The IRC recommends a minimum desirable width of 5.0 m for medians of rural highways, which may be reduced to 3.0 m where land is restricted. On long bridges the width of the median may be reduced up to 1.2 to 1.5 m. The medians should normally be of uniform width on a particular road, but where change in width is unavoidable, a transition of 1 in 15 to 1 in 20 must be provided.

8. Thus in highways with divided carriageway, the total road width depends on the width of pavement or carriageway and the width of the median/dividing island/traffic separator. On urban highways with six lanes or more, medians should invariably be provided.

9. The minimum recommended width of medians at intersections of urban roads are 1.2m for pedestrian refuge, 4.0 to 7.5 m for protection of vehicles making right turn and 9.0 to 12 m for protection vehicles crossing at grade . The absolute minimum width of median in urban area is 1.2 m and desirable minimum is 5.0 m.

10. Kerbs Kerb indicates the boundary between the pavement and median or foot path or island or shoulder. It is desirable to provide kerbs on urban roads. Refer Fig. 4.3. There are a variety of kerb designs.

11. Kerbs may be mainly divided into three groups based on their functions. ( a) Low kerb ' or 'mountable type kerb ‘ encourages traffic to remain in the through traffic lanes, yet allows the driver to enter the shoulder area at slow speed. The height of this type of kerb is about 100 mm above the pavement edge with a slope or batter to enable vehicles to climb the kerb at slow speed. This type of kerb is provided at medians and channelization schemes and is also useful for longitudinal drainage system

12. (b) `Semi-barrier type kerb ' is provided on the periphery of a roadway where the pedestrian traffic is high. This type of kerb has a height of about 150 mm above the pavement edge with a batter of 1:1 on the top 75 mm. This kerb prevents encroachment by the parking vehicles, but at acute emergency it is possible to drive over this kerb with some difficulty

13. (c) Barrier type kerb is provided in built-up areas adjacent to foot paths with considerable pedestrian traffic. The height of kerb stone is about 200 mm above the pavement edge with a steep batter of 1.0 vertical 0.25 horizontal

14. At times submerged kerb is provided on rural roads between pavement edge and shoulders. The submerged kerb provides lateral confinement and stability to the flexible pavements in general and to the granular base course of pavements in particular.

15. Road Margins The various elements included in the road margins are shoulder, guard rail, footpath, drive way, cycle track, parking lane, bus bay, lay-bye, frontage road and embankment slope.

16. Shoulders The important functions of shoulders are: (a) The capacity of the carriageway and the operating speeds of vehicles increase if the shoulders are laid and maintained in good condition

17. (b) Shoulders serve as emergency lanes for vehicle compelled to be taken out of the main carriageway or roadway. Shoulders should have sufficient load bearing capacity to support loaded truck even in wet weather

18. (c) Shoulders also act as service lanes for vehicles that are disabled. The width of shoulder should be adequate to accommodate stationary vehicle fairly away from the edge of adjacent lane.

19. Guard rails Guard rails are provided at the edge of the shoulder when the road is constructed on a fill so that vehicles are prevented from running off the embankment, especially when the height of the fill exceeds 3 m. Various designs of guard rails are in use. Guard stones (painted with black and white strips) are installed at suitable intervals along the outer edge of the formation at horizontal curves of roads running on embankments along rural areas so as to provide better night visibility of the curves under head lights of vehicles.

20. Footpath or side-walk : Min width : 1.5 m to 2.0m. Drive ways : connect the highway with commercial establishment like fuel-stations service-stations. Cycle tracks : min. width 2m provided. Parking lane : min. lane width 3.0m.

21. Embankment slopes Embankment slopes should be as flat as possible for the purpose of safe traffic movement and also for aesthetic reasons. Though from the slope stability point, a steeper slope may be possible, the slope may be kept as flat as permitted by financial considerations. Road side landscaping can improve the aesthetic features of road side, making road travel more pleasant. For safety considerations, the desirable slope for the embankment is 1 in 3

22. Fig : Cross section details

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