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As mentioned earlier, each intersection has a unique geometry and flow values,
and hence needs special attention as far as the use of Channelization devices
are concerned.
The main factors affecting the selection of the island type are:
- Traffic characteristics at the intersection
- Cost considerations, and
- Maintenance needs
The raised islands and flush Channelization are dealt with in details in the
following sections.
Flush Channelization is usually appropriate in the following conditions:
- On high speed rural highways to separate turning lanes.
- In constrained locations, i.e. the locations where vehicle path definition
is desired but space for raised islands not available.
- For separating opposing traffic streams of low speed streets.
- In areas where frequent removal of snowfall is required, i.e. in places of
high snow fall.
- It can also be used as a temporary Channelization either during
construction or to test traffic operations prior to the actual installation of
raised islands.
However, the main demerits of this type of Channelization are :
- It is not effective in prohibiting or preventing traffic movements.
- It is also not appropriate for islands intended to serve as pedestrian
refuge.
The locations where the construction of raised islands assumes importance are:
- The primary function of the channelizing device is shielding pedestrians
or to provide refuge to pedestrians crossing a street.
- Also, the primary/secondary function is locating traffic signals or other
fixed objects.
- Intention is to prohibit or prevent certain traffic movements.
- To separate high volume opposing traffic flows.
- The raised islands are also particularly important at intersections with
unusual geometry i.e. skewed intersections.
A comparison between the usefulness and the operating conditions of the two
types of Channelization is presented in Table. 1.
Table 1:
Flush Channelization vs Raised Channelization
| FLUSH CHANNELIZATION |
OPERATING CONDITIONS |
|---|
| 1. For Right turns |
High Speeds |
| 2. To provide temporary or trail Channelization |
Rural highway |
| 3. To shadow left turns |
Minor urban intersections |
| RAISED CHANNELIZATION |
OPERATING CONDITIONS |
|---|
| 1. Post signs or signals |
Urban streets |
| 2. Provide pedestrian refuse |
Low speeds |
| 3. Prevent wrong way movements |
High volumes |
The main design principles followed for the design of the shape and size and
shape of the traffic island are as follows:
- Shape and size: Islands are generally either narrow and elongated
or triangular in shape, are normally situated in areas of the roadway outside
the planned vehicle paths, and are shaped and dimensioned as component parts of
the street or intersection layout.
The actual size differs as governed by site conditions, but the following
minimum size requirements should be met to insure that the island will be large
enough to command attention.
- Traffic lanes or turning roadways should appear natural and convenient to
their intended users.
- Number of islands should be held to a practical minimum to avoid
confusion.
- The islands should be large enough to be effective. Small islands do not
serve as channelizing devices and pose maintenance problems.
- These should not be introduced at locations with restricted sight distance
or middle of sharp horizontal curves due to sight distance considerations.
Table. 2 gives the recommended minimum and desired area
values of the traffic islands in typical urban and rural intersections.
Table 2:
Recommended Island Sizes
| Location of Intersection |
Size(Sq.meters) |
| |
Minimum |
Desired |
| Urban |
4.65 |
7 |
| Rural and High Speed urban/Suburban |
7 |
9.3 |
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