Facts of the Matter


Sunday, December 16, 2001

Studies dispute links
between speed, accidents

The controversy about cameras catching speeders in the act begs the question of whether there is actually a relationship between speed and traffic accidents.

Common sense suggests that higher speeds lead to more accidents and a more fatalities, and that increasing speed limits cause drivers to drive faster.

Studies by the National Highway Traffic Safety Administration and the Federal Highway Administration show exactly the opposite in both cases.

The NHTSA study found that speeding has been implicated as a contributing factor in about one-third of all fatal motor-vehicle crashes. But NHTSA's definition of "speed related" includes:

1) improper lane changes,
2) following too closely,
3) unsafe passing,
4) inattention,
5) reckless driving,
6) high speed chase,
7) erratic speeds,
8) driving too fast for conditions not necessarily above the limit, and
9) driving less than posted minimums.

The truth is that only 5 percent of fatalities are due to "erratic" speed, irrespective of speed limits.

The Federal Highway Administration conducted a seven-year study in which speed limits were varied at 100 locations in 22 states and the resulting traffic flow analyzed. It concluded that speed limits have little or no effect on the average speed of traffic: " ... Raising posted speed limits by as much as 15 mph had little effect on motorists' speeds ... contrary to public perception, the data actually indicate that accident rates were reduced at sites where speed limits were raised."

A New York State Department of Transportation report on the increase of speed limits from 55 mph to 65 mph found that the fatal accident rate decreased by 29 percent, the total accident rate decreased by 4 percent and the injury accident rate decreased by 5 percent.

A study carried out at the University of California at Irvine compared the fatality rates between states that adopted the 65 mph speed limit on interstate highways and those that stayed at 55 mph.

The researchers found that "... overall fatality rates fell by 3.4 to 5.1 percent in the states that adopted the 65 mph limit."

The FHA study also found that the majority of speed limits are posted well below the average speed of traffic. They concluded that lowering speed limits below the 50th percentile does not reduce accidents, but does significantly increase driver violations of the speed limit.

Conversely, raising the posted speed limits did not increase speeds or accidents. These facts suggest to some that speed limits have become largely irrelevant as a source of guidance to motorists and impractical as a threshold for enforcement purposes.

These studies suggest that the reason for much of the traffic speeding along at 15 mph over the limit today is because the limit is set way to low!

The speed at which 85 percent of the vehicles are traveling at, or below, has generally been determined to be a limit that minimizes accident risk and maximizes motorist compliance. It blends an optimum combination of efficiency, consensus, enforceability and safety.

According to legislation passed in Michigan (the Michigan Fair Motoring Act), studies showed that speed limits that are set below the 85th percentile speed:

>> Do not yield an increase in safety.
>> Create a greater variation of vehicle speeds on a given roadway.
>> Are generally ignored by motorists unless extreme enforcement is used.

Honolulu traffic authorities claim that their recent survey showed 30 percent of Hawaii motorists exceeding posted speed limits. This puts our speed limits at best at the 70th percentile, and supports the perception that Hawaii's speed limits are too low. This leads to the conclusion that either Honolulu transportation officials are ignorant of the reports and recommendations, or that speed limits are purposely set too low in order to generate revenue in the form of a speed tax.

This is only one example of how statistics can be used to draw invalid conclusions. Numbers alone do not speak the truth, and they must be interpreted correctly to be meaningful.

Folk wisdom says that it is easy to lie with statistics. A colleague of mine says that it is easier to lie without them.

We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at

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