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Basic tenets of traffic safety

October 13, 2012
By DAVE WERNER ( , Franklin County Traffic Safety Board

Past "Did You Know" articles have dealt at length with proper setting of speed limits on our highways. If you are a frequent reader of these articles, you are aware of the importance of the 85th percentile, the speed at or below which 85 percent of the vehicles are traveling. I recently came upon an interesting report by Wade-Trim, a consulting firm specializing in improving roadway safety.

According to this report, speed limits are primarily set for safety reasons - i.e., to reach a balance between travel time and crash risk - and to provide a basis for enforcement of inappropriate speeding behavior. Maximum speed limits enhance safety by placing an upper limit on speed choice, and reduce the differences in vehicle speeds by drivers using the same road at the same time.

Based on years of experience and observation, the report continues, the following fundamental concepts have been used to establish realistic speed zones.

The majority of motorists drive at a speed they consider reasonable and safe for road, traffic and environmental conditions. Posted limits which are set higher or lower than dictated by roadway and traffic conditions are ignored by the majority of motorists.

The normally careful and competent actions of a reasonable person should be considered legal. We shouldn't be ticketing motorists for speeding if the road is designed to be driven on at a speed well above the posted limit.

A speed limit should be set so that the majority of motorists observe it voluntarily and enforcement can be directed to the minority of offenders.

A driver's choice of speed can impose risks on other road users. Crash severity increases with increasing speeds because in a collision, the amount of kinetic energy dissipated is proportional to the square of the velocity. Crashes, however, appear to depend less on speed and more on the variation in speeds. The likelihood of a crash is significantly greater for motorists traveling slower or faster than the mean speed of traffic.

Maximum speed limits are set for ideal road, traffic and environmental conditions.

Establishing safe and realistic uniform speed zones is important, according to the report, because it invites public compliance by conforming to the behavior of the majority of motorists and provides a clear reminder to violators. It also assists the courts by providing a guide as to what constitutes a reasonable and prudent speed and reduces arbitrary enforcement and conviction tolerances.

Think about the above, and put your driving in perspective. Won't you agree that you drive according to the above conditions? Do you agree that if the road is conducive to driving at 50 mph but is posted for 30 mph, you tend to drive well above the posted speed limit?

Good engineering practice now says that highway engineers should design our streets and roads in such a manner that the desired speed is controlled by the road's design rather than posted speed limits. In other words, if you want the speed to be 30 mph, don't offer straight roads with wide driving lanes and shoulders. Narrow it down, put in chicanes and islands, plant trees, and most drivers will travel at this desired speed. And the big benefit is that heavy enforcement is not necessary.


For more articles on traffic law and safety, go to the traffic safety board's website at and click on "Traffic Safety Board" under departments then look for Did You Know articles under "services."



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