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Decision Sight Distance
Normally, the stopping sight distance is an adequate sight distance for
roadway design. However, there are cases where it may not be appropriate. In areas
where information about navigation or hazards must be observed by the driver, or where the
drivers visual field is cluttered, the stopping sight distance may not be adequate.
In addition, there are avoidance maneuvers that are far safer than stopping, but require
more planning by the driver. These may not be possible if the minimum stopping sight
distance is used for design. In these instances, the proper sight distance to use is the
decision sight distance.
The decision sight distance is the distance traversed while recognizing an object or
hazard, plotting an avoidance course, and making the necessary maneuvers. Unlike the
stopping sight distance, the decision sight distance is quite complex. Various design
values for the decision sight distance have been developed from research. The table
below gives a few values for the decision sight distance (AASHTO, 1994).
Design Speed (km/h) |
Decision Sight
Distance (meters) |
Stop
Rural
Road |
Stop Urban Road |
Adjustment Rural
Road |
Adjustment Suburban Road |
Adjustment Urban Road |
50 |
75 |
160 |
145 |
160 |
200 |
80 |
155 |
300 |
230 |
275 |
315 |
90 |
185 |
360 |
275 |
320 |
360 |
110 |
265 |
455 |
335 |
390 |
435 |
It is up to the engineer to decide when to use the
decision sight distance. Providing the extra sight distance will probably increase
the cost of a project, but it will also increase safety. The decision sight distance
should be provided in those areas that need the extra margin of safety, but it isnt
needed continuously in those areas that don't contain potential hazards.
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