Going with 85% of the flow

Times Staff Writer

To the uninitiated, the “85th percentile rule” seems bizarre, unorthodox and maybe even scary, but this speed limit benchmark has guided traffic engineers for decades and is even recognized as official policy in many government jurisdictions.

The idea is that maximum speed limits should be set so that 85% of the vehicles on a particular stretch of road are at or below the limit. Under California policies, traffic engineers routinely measure how fast motorists drive and then often set the limit at the 85th percentile of traffic speed.

I wonder what would happen if the Internal Revenue Service only collected taxes based on what 85% of people actually pay. Fast-food restaurants might fry hamburgers so that 85% of people don’t get sick. And, of course, 85% of Southern Californians should buy the Los Angeles Times and others should get tickets.


Only in the traffic world does the 85th percentile rule make any sense. The underlying concept is that 85% of drivers are sane and safe individuals, while the other 15% are maniacs who should be ticketed. No kidding.

“The reasoning is that 85% of people drive reasonably and 15% do not,” said David Roseman, city traffic engineer for Long Beach. “So we should be designing our speeds to accommodate reasonable drivers.”

Adds Tom Jones, principle traffic engineer for the city of Los Angeles, “The 85th percentile rule was established many years ago. It is a design criteria, but it doesn’t mean that it is necessarily OK.”

Safety advocacy groups hate the 85th percentile rule, because they believe that speeding is a serious and growing highway hazard. Indeed, police are performing fewer routine traffic patrols and speeds are creeping up, according to studies published by safety groups. Barbara Harsha, executive director of the Governors Highway Safety Assn. in Washington, for example, is concerned that the 85th percentile rule can be used to legitimize unsafe speeding.

When congestion is not limiting speeds, many sections of Southern California freeways have average speeds of more than 80 mph, well above the legal limit. Posted limits of 25 mph on residential streets are routinely ignored, according to neighborhood traffic studies.

That just shows that legal speed limits are too low, says Chad Dornsife of the National Motorists Assn., a group that represents people generally unhappy with and often indignant about traffic laws and police enforcement. “I have seen speed limits set at the zero percentile, where 100% of the vehicles are speeding,” Dornsife said.


He says improperly set low speed limits actually increase accidents and cost lives, because they encourage unequal speeds, creating a hazard. He claims, for example, that when Montana imposed speed limits for the first time, fatal accidents doubled.

A secondary problem Dornsife cites regarding artificially low speed limits is that yellow-light intervals are sometimes based on the posted limits, which leaves too little time for faster-moving cars to stop for a changing light before reaching an intersection. That, Dornsife claims, creates intersection collisions.

“Every generation that has gone through this doesn’t believe in the 85th percentile rule,” he adds. “The law-enforcement community doesn’t like the 85th percentile rule because they write fewer tickets. New traffic engineers aren’t even taught the 85th percentile rule.”

The rule’s logic has a distinct appeal: It seems democratic. Why should a government bureaucrat get to overrule what 85% of the people do in a democracy, including how fast you can drive your car?

Of course, there are many reasons why traffic engineers are in a better position to judge what is safe and reasonable than motorists trying to get their screaming kids to a soccer game on time. By the logic of the 85th percentile rule, you might eliminate stop signs and traffic lights entirely.

“There are many instances where the 85th percentile rule is not appropriate,” said Jones, the L.A. traffic engineer. “There are reasons to consider other than the 85th percentile rule, such as the proximity of schools and the need to provide safety for pedestrians.”


Of course, the 85th percentile rule does not mean that 85% of all cars drive at the same maximum speed limit. Most drivers within the 85th percentile are actually going slower than the limit, so you will always have the problem of mixed-speed use on roads. Harsha said studies have shown that increasing speed limits does not create more uniform speeds and in fact can have a negative effect on safety, though the results are not all that clear.

The 85th percentile rule might make more sense if the road culture in this country were not deteriorating to such a low common denominator. Rude, obnoxious and unsafe behavior seems to be more common every day: Drivers cutting off other motorists, people zig-zagging 100 mph down the freeway and the frequent flipping of obscene gestures.

But maybe those people are in the 15th percentile that will never obey speed limits, no matter how high they are set.

Ralph Vartabedian can be reached at ralph.vartabedian