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Road Management & Engineering Journal |
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April, 2000 TranSafety, Inc. (360) 683-6276 Fax: (360) 683-6719 |
A realistic speed limit provides more benefit to a road than an unrealistic speed limit.
Simply dropping the speed limit will probably do nothing to "fix the problem."
An unrealistically low speed limit can actually cause more harm than good. In fact, it
can actually lead to more crashes.
Here's why:
First, many studies conducted over several decades in all parts of the country show
that a driver's speed is influenced more by the appearance of the roadway and the
prevailing traffic conditions than it is by the posted speed limit.
Second, some drivers will obey the lower posted speed while others will feel that it is
simply unreasonable and simply ignore it. This disrupts the uniform traffic flow and
increases crash potential between the faster and slower drivers.
Third, when traffic is traveling at different speeds, the number of breaks in traffic to
permit safe crossing of vehicles or pedestrians is reduced. Pedestrians have greater
difficulty in judging the speed of approaching vehicles.
The basic intent of speed limits is to identify a safe and reasonable limit for a particular
section of road.
The most widely accepted method is to set the limit at a speed which 85% of the traffic
is moving at or below. This reflects the safe speed as determined by a large majority of
the drivers. Research has shown that this "85th percentile" speed is the speed where
accident involvement is the lowest. Reducing the limit below that 85th percentile will
probably be detrimental to safety.
