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Sight distance available from a point is the actual distance along the road
surface, over which a driver from a specified height above the carriage way has
visibility of stationary or moving objects.
Three sight distance situations are considered for design:
- Stopping sight distance (SSD) or the absolute minimum sight distance
- Intermediate sight distance (ISD) is defined as twice SSD
- Overtaking sight distance (OSD) for safe overtaking operation
- Head light sight distance is the distance visible to a driver during
night driving under the illumination of head lights
- Safe sight distance to enter into an intersiection.
The most important consideration in all these is that at all times the driver
traveling at the design speed of the highway must have sufficient carriageway
distance within his line of vision to allow him to stop his vehicle before
colliding with a slowly moving or stationary object appearing suddenly in his
own traffic lane.
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