Many states use the “85% rule” to set speed limits. The idea is simple: track how fast cars drive on a road, find the speed at which 85% of drivers are at or below, then set the limit within 5 mph of that.

It’s quick. It’s easy. But critics argue it’s flawed.

The rule doesn’t account for regional driving habits. In rural states, drivers often creep along two-lane highways. Police may stop you for going eight miles over the limit. In Los Angeles or New Jersey, the same pace will earn you honks for going too slow—even 10 mph above the posted limit.

Jenny O’Connell, director of member programs for the National Association of City Transportation Officials, doesn’t like the rule. “The problem with this approach is it creates this feedback loop… People speed, and then the speed limits will be ratcheted up to match that speed.”

Cities push back against higher limits

Charlotte, North Carolina, has challenged the 85% rule. Seattle and Madison, Wisconsin, already lowered downtown limits and saw fewer crashes.

Many European cities take it further, embracing a “Vision Zero” policy that treats every traffic death as unacceptable. Helsinki cut speed limits to below 20 mph on half its city streets. Last year, the city recorded zero traffic deaths.

What comes after the 85% rule?

The U.S. may follow suit, but only with stronger public transit to balance lower limits. Highways could also be re-evaluated. Current speed limits were set decades ago, when vehicles were less safe and far slower.