Module 6 : Traffic Intersection Control
Lecture 31 : Channelization
1 2 3 4 5 6 7 8 9 10 11 12 13
 

Traffic Islands

A principle concern in Channelization is the design of the islands. An island is a defined area between traffic lanes for control of vehicle movements. Within an intersection area, a median or an outer separation is considered to be an island. It may range from an area delineated by barrier curbs to a pavement area marked by paint.

Classification of Islands

Traffic islands usually serve more than one function, but may be generally classified in three separate types:
  1. Channelizing Islands - These are designed to control and direct traffic movement, usually turning. Channelizing islands are are shown in Fig. 1.
    Figure 1: Channelizing Islands
    \begin{figure} \centerline{\epsfig{file=qfChanChannelizingIslands.eps,width=8cm}} \end{figure}
  2. Divisional Islands - These are designed to divide opposing or same direction traffic streams, usually through movements. Fig. 2 shows the placing of divisional islands in a roadway.
    Figure 2: Divisional Islands
    \begin{figure} \centerline{\epsfig{file=qfChanDivisionalIslands.eps,width=8cm}} \end{figure}
  3. Refuge islands - Pedestrian islands are provided to serve as safety zones for the aid and protection of persons on foot. If a divisional island is located in an urban area where pedestrians are present, portions of each island can be considered a refuge island. Refuge islands are shown below I Fig. 3.
    Figure 3: Refuge Islands
    \begin{figure} \centerline{\epsfig{file=qfChanRefugeIslands.eps,width=8cm}} \end{figure}
    The design aspects of the traffic islands are dealt in detail in the following sections.

Design Considerations for Traffic Islands

The necessity for an island should be determined only by careful study, since it is placed in an area that would otherwise be available for vehicular traffic. The island design should be carefully planned so that the shape of the island will conform to natural vehicular paths and so that a raised island will not constitute a hazard in the roadway. A judiciously placed island at an intersection on a wide street may eliminate the need for traffic signal control by channelizing traffic into orderly movements. The total design of traffic islands can be studied in three steps:
  1. Selection of appropriate island type (barrier, mountable, painted or flush): The site and traffic conditions in each intersection are different and hence the island type suitable for each requires separate attention. The traffic island selected may vary from barrier type islands to flush islands marked on the roadway surface.
  2. Determination of shape and size of islands: The shape of the island and its size in an intersection depends on the geometry and space availability at the same. A proper shape and size of the island (in case of raised islands) must be selected so that it is able to both channelized the traffic and not pose any type of hazard.
  3. Location relative to adjacent traffic lanes: The islands must be offset from the roadway by some distance to remove the risk of a vehicle dashing against the same. The width of offset is maximum at the entry of the island and decreases gradually as one moves towards the end of it.