kerb

Kerbs indicate the boundary between the carriage way and the shoulder or islands or footpaths. Different types of kerbs are
Low or mountable kerbs : This type of kerbs are provided such that they encourage the traffic to remain in the through traffic lanes and also allow the driver to enter the shoulder area with little difficulty. The height of this kerb is about 10 cm above the pavement edge with a slope which allows the vehicle to climb easily. This is usually provided at medians and channelization schemes and also helps in longitudinal drainage.

Semi-barrier type kerbs : When the pedestrian traffic is high, these kerbs are provided. Their height is 15 cm above the pavement edge. This type of kerb prevents encroachment of parking vehicles, but at acute emergency it is possible to drive over this kerb with some difficulty.

Barrier type kerbs : They are designed to discourage vehicles from leaving the pavement. They are provided when there is considerable amount of pedestrian traffic. They are placed at a height of 20 cm above the pavement edge with a steep batter.

Submerged kerbs : They are used in rural roads. The kerbs are provided at pavement edges between the pavement edge and shoulders. They provide lateral confinement and stability to the pavement. ( Courtesy: http://www.cdeep.iitb.ac.in )

More..

Kerbed roads have a significant affect on driver behaviour. Kerbing affects the distance that drivers align heir vehicles from the edge of a road and acts as a physical and psychological barrier that discourages them from leaving the road surface. Generally lateral placement of vehicles varies with kerb height and steepness as well as the location of other obstructions outside the kerb-line. Kerbing improves delineation of road edges and contributes to the appearance and safety of the road.The main purposes for kerb construction are:

      • To assist drainage.
  • To improve channelisation and delineation of traffic flows.
  • To protect pedestrians.
  • Improvement of aesthetic values of the road alignment.
  • To reduce maintenance of shoulders.
    • To provide a boundary to landscaping treatments.


The use of kerbing to control vehicle movements is generally recommended in urban environments particularly at intersections because it is most effective at low speeds and small angles of impact. Kerbing is not recommended on rural roads and on high-speed roads, unless specifically required to improve channelisation at rural intersections, to control drainage in cuts, on bridges etc.

Construction of kerbing may significantly affect the cost of a project. Therefore, construction of kerbing should be justified in the following situations:

  • Limited road reserve (introduction of kerbing reduces the cross-section width).
  • Deep cuttings (to assist drainage).
  • Special environmental requirements (control accidental spills, drainage of bridges etc.).
  1. Structural requirements (to protect high embankments from scouring)

 

3. TYPE OF KERBING

The two basic classes of kerbs are barrier and mountable kerbs.  Each class has different types including semi-mountable and semi-barrier kerbs with a variety of designs (Figure 2). Dimensions of the most common types of kerbing used on Western Australian roads are detailed in Drawing No 9331-0376.


Figure 2 – Type of Kerbs

3.1 Barrier Kerb
Barrier kerbs are steep-faced and are designed to prevent vehicle encroachment on the roadside. Their main functions are:                                                                              

      • to discourage vehicles from using areas outside the travelled way, not intended for vehicular travel;
  • to control drainage;
  • to control parking of vehicles;
    • to reduce the risk to pedestrians.


The typical barrier kerb is 150mm high. This height is effective to prevent vehicle encroachment into the roadside at low to moderate speeds.

Barrier and semi-barrier kerbing should generally be avoided on freeways or highways with design speeds of over 70 km/h because impact with kerbing on high-speed roads may overturn a vehicle or result in a vehicle becoming airborne.

Barrier-type kerbs may be used on sections of road where separation of opposing traffic is essential due to the high safety risks associated with traffic volumes, percentage of heavy vehicles, speed, crash history etc.

 

If a design allows for kerbed shoulders with barrier type kerbing the shoulder width should be adequate to accommodate a disabled vehicle (eg. AASHTO recommends a min of 1.8m; Austroads recommends a minimum width of 2.0m). The desirable shoulder widths for rural and outer urban roads are shown in Main Roads Supplement to Austroads GRD Part 3: Geometric Design – Table 4.3.

Barrier kerb, if positioned along the edge of a traffic lane, gives drivers a sense of restriction. Also large vehicles travelling along kerbed carriageways have no additional space in which to manoeuvre or to allow for sway of the rear trailer. It is recommended to avoid this type of kerbing on roads with restricted lane width and high percentage of heavy vehicles.

Barrier kerbs reduce the risk to pedestrians, not only as a physical but psychological barrier as well, because drivers generally tend to shy away from the kerb line.  For this reason, barrier kerbing is recommended in built-up areas adjacent to footpaths with considerable pedestrian traffic, shared use paths and also at bus bays.

Barrier-type kerb may be used on low speed (< 70 km/h) arterial roads in order to prevent mid-block turns.

Some of the above text was adopted from AASHTO’s Policy on Geometric Design of Highways and Streets (2004).

3.2 Semi-Barrier
This type of kerbing is recommended where pedestrian traffic is light and a barrier type could tend to reduce traffic capacity due to the impression of restriction.

 

3.3 Mountable
Mountable kerbs (Type A and M) are generally used in the following situations:

      • At the outer mountable island area of intersections, small corner islands and roundabouts to outline standard vehicle travelled paths.
  • To define the left edge of a through carriageway where the crossfall of the adjacent shoulder or parking strip is opposite to that of the through carriageway.
  • Where crossing or encroachment by vehicles larger than the design vehicles is permitted (e.g. at roundabouts) or expected under emergency conditions.
  • In front of road safety barriers.
    • On pedestrian and cycle paths along the grassed edge of asphalt paths to reduce damage to the path from the grass growing into the asphalt path.  Kerbing along paths also provides visual contrast to the path edge and prevents the verge material erosion onto the path.

Flush type kerb (refer to Type M in Figure 1) are generally used in the following situations:

    • On lightly travelled residential streets because it does not require modification at driveway entrances.

The design of mountable kerbs should not result in loss of vehicle control or undercarriage damage when struck.

 

3.4 Semi-Mountable
Semi-mountable kerbing should be used at all intersections, junctions and island treatments and is often used on outer separators and raised medians on bridges.Semi-mountable kerb may also be used along pedestrian and cycle paths.

 

3.5 Other
The following   types of kerbs are used to enhance the visibility/perception of a road alignment:

    • High-visibility kerbing may be specified by a designer in areas where visibility is restricted due to fog or extended rains. For this purpose white marking or white cement may be used as per AS 1742.2 – 1994.
    • Reflective kerbs use retro-reflective markings to improve visibility/perception. They are advantageous at night if placed along through-lane edges and island kerbs at intersections with high percentage of night-time crashes.

Where such alternative kerb types are proposed to be used on roads managed by Main Roads, approval from the Main Roads Project Manager shall be obtained.


Courtesy: https://www.mainroads.wa.gov.au/

Articles:

 

Did you like this? Share it: