- The 85 percent rule sets speed limits by driver behavior, but critics say it’s unsafe.
- Some states and cities are now shifting to context- and safety-based approaches.
- Street design often influences driver speed more than the number on the sign.
If you’ve ever been on a road where the speed limit feels artificially low or surprisingly high, you’re not alone. For decades, many states have leaned on a single formula to decide how fast cars should travel.
var adpushup = window.adpushup = window.adpushup || {que:[]};
adpushup.que.push(function() {
if (adpushup.config.platform !== “DESKTOP”){
adpushup.triggerAd(“0f7e3106-c4d6-4db4-8135-c508879a76f8”);
} else {
adpushup.triggerAd(“82503191-e1d1-435a-874f-9c78a2a54a2f”);
}
});
Now, after years of accidents, lots of close calls, and countless complaints, some states are rethinking their strategy. Even the NHTSA has joined the chorus urging drivers to slow down. Yet in all this debate, one major detail may be slipping through the cracks.
The Speed Formula
At the center of it all is a formula known as the 85 percent rule, which assumes the safest speed on a road is the one most drivers naturally choose. Keep that in mind, as it’ll prove important later on. Engineers study free-flowing traffic, identify the speed at or below which 85 percent of vehicles travel, and round it to the nearest five miles per hour. The approach has guided speed limits for decades, but critics argue it is outdated and unsafe.
Read: Trucker’s Rogue U Turn In Florida Ended In A Tragedy
Safety advocates argue that the rule creates a self-reinforcing cycle: people drive faster, the limit gets raised to match, and roads become more dangerous. That goes for everyone, including pedestrians, cyclists, and even drivers themselves.
Federal Shift in Guidance
According to the Associated Press, the federal government recently clarified that the 85% figure was never meant to be a rule at all, encouraging states to weigh other factors like crash data, land use, and pedestrian activity.
var adpushup = window.adpushup = window.adpushup || {que:[]};
adpushup.que.push(function() {
if (adpushup.config.platform !== “DESKTOP”){
adpushup.triggerAd(“bb7964e9-07de-4b06-a83e-ead35079d53c”);
} else {
adpushup.triggerAd(“9b1169d9-7a89-4971-a77f-1397f7588751”);
}
});
Some cities have already changed things up. Madison, Wisconsin, is rolling out its “20 is Plenty” campaign, which lowers the limit from 25 mph to 20 in many residential areas. Meanwhile, Seattle saw a serious injuries decline after lowering neighborhood limits to 20 mph.
Ohio now allows municipalities to consider the slower 50th percentile speed in busy pedestrian areas. Of course, the system doesn’t seem to work everywhere. Boulder, Colorado, authorities say it did little to reduce speeds.
Why the Old Rule Persists
Perhaps that’s why many states stick with the simplicity of the old system. They’re supported by driver advocates who argue that speed limits should reflect how roads are actually built to be driven. “It doesn’t really matter what number you put on a sign,” says Jay Beeber of the National Motorists Association. “The average driver drives the nature of the roadway.” That’s an important point.
var adpushup = window.adpushup = window.adpushup || {que:[]};
adpushup.que.push(function() {
if (adpushup.config.platform !== “DESKTOP”){
adpushup.triggerAd(“b25ecba7-3bbb-4ea7-a3a8-dbea91695c07”);
} else {
adpushup.triggerAd(“e46c436a-adeb-4b5e-a2c7-56bc36561c10”);
}
});
Street design itself often dictates behavior far more than a posted number. Wide lanes, sweeping curves, and long sightlines invite higher speeds, whether the sign says 35 or 55. “It would be patently unfair for a government to build a road to encourage people to drive 45 mph, put a 30 mph speed limit on it, and then ticket everyone for doing what they built the road to do,” said Beeber.
Narrow lanes, tighter turns, or visible pedestrian activity, by contrast, naturally slow drivers down. If states want safer roads, rethinking how streets are built is likely far more important than how speed limits are set.