Road Management & Engineering Journal
Road Management & Engineering Journal
April, 2000
TranSafety, Inc.
(360) 683-6276
Fax: (360) 335-6402

Lower Speed Limits May Cause More Problems Than They Solve

(This article is reproduced, with permission, from the Spring 1999 issue of Interchange, the newsletter of the Nebraska Technology Transfer Center. Interchange reprinted the article with permission from the Maine Local Roads News for December 1998. The Maine Local Roads News also granted TranSafety, Inc. permission to reprint.)

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.

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