The Traffic Accident Reconstruction Origin -ARnews-

Re: Sight Distances

Zygmunt M. Gorski (gorski@golden.net)
Mon, 19 Oct 1998 15:36:06 -0400 (EDT)

The American Association of State Highway Transportation Officials
(AASHTO) publishes a manual entitled "A Policy on Geometric Design of
Highways and Streets" (1994) in which the guidelines for visibiity
distances are discussed for vertical alignments (crests sags),
horizontal alignment (visibility around a curve) passing
sight distance and stopping sight distance.

For roadway "dips" or sags there are standard guidelines typically
assuming a driver's eye height is 105 centimetres above the ground
and that a target, perhaps 15 centimetres above the ground must be
visible over a certain distance (135 metres) on a highway of a specific
design speed (80 km/h). These numbers refer to stopping sight distance.
I believe you may still obtain a copy of the manual in imperial
(english) units.

Passing sight distance is a separate guideline and refers to the distance
that a passing vehicle requires to complete a safe passing motion. Since
there is an approaching vehicle involved the visibility distance is much
greater, I believe in the range of 540 metres for a road design speed of
80 km/h. Other factors such as roadway grade need to be taken into
consideration. For passing sight distance the height of the observed
object be seen is set at 130 centimetres.

Although these guidelines may be useful for design, the character of the
site still needs to be assessed in terms of its specific effect on the
collision consequences. It may be found that a site does not meet certain
guidelines but the effect on the collision consequences may be minimal.

There are separate guidelines

Zygmunt M. Gorski
gorski@golden.net

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