Photo: Jeff Roberson (Associated Press)

You, a human being who we imagine likes to drive cars, are probably aware of the signs posted that state exactly how fast you can legally drive on a given road. But how the hell do those things even get determined?

The simplest, obvious answer to how speed limits are set is that, duh, lawmakers make the decision. But there’s a reason why they settle on a certain number, and it has to do with engineering, data collection, average speeds and even collisions.

85th Percentile

Speed limits have an objective of ensuring that drivers travel efficiently and safely. Engineers can figure out how to achieve both points by conducting a speed study, which takes into account several data points.


It covers a lot, as a transportation columnist for Washington’s Bellingham Herald noted a few years back:

A speed study consists of a review of the function, design and actual use of the road. Is the road intended for heavy commuter traffic or light local traffic? How wide are the lanes? How wide are the shoulders? Is it straight or curvy? Are there a lot of intersections? These and many other questions about the road get answered in the study.

From here, things get super math-involved.

Our hypothetical engineer figures out how many drivers are on the road and how fast they drive. Once that data’s collected, it’s plotted out to help determine how fast the majority of drivers are traveling. In particular, the engineer wants to determine the “85th percentile speed”—meaning how fast 85 percent of the cars travel.


The 85th percentile speed, a blogger at engineer company SEH put it, is called the “prevailing speed,” because it’s considered the safest speed to travel. Interesting, right?

There’s more, though!

The engineer then determines how many collisions occur on a specific road, the rate at which they occur, and then compares that number to the overall region, according to the Bellingham Herald columnist. (Collission rates are measured as how many crashes happen per million miles traveled.)


Once this amalgamation of data is united, the engineer sets about determining some sort of recommendation.

It seems fairly easy to come to a conclusion from here, per the Bellingham Herald:

If the crash rates are high and the 85th percentile speed is at or near the posted speed limit, the engineer likely would recommend reducing the speed limit. If the crash rate is high but the 85th percentile speed is significantly higher than the posted speed limit, the recommendation might be for additional enforcement of the current speed limit. If the crash rate is low and the 85th percentile speed is much higher than the posted speed limit the engineer could recommend increasing the speed limit.


Speed surveys can be rounded up in 5 mph increments, notes the Sacramento Bee, which interviewed an engineer for the City of Lincoln, California, back in 2015 about setting speed limits.

If the 85th percentile speed is 34 mph, the Bee reports, they’ll likely bump up the limit to 35 mph, but “they can round down to 30 if they determine there is a particular safety issue.”

‘Speed Kills’

Now, in an effort to avoid a serious, meandering digression, it’s worth a brief mention that not everyone agrees with the 85th percentile speed as a solid measure to determine the speed limit.


For one thing, law enforcement agencies make bank on speeding tickets. I remember hating a particular stretch in my hometown that was a well-known speed trap; the conditions along the road constantly left drivers traveling at a—reasonably—faster speed, and cops constantly nailed them for driving too fast. (My hometown once had a reputation as the worst city for speed traps, too.)

And as this takedown of the “speed kills” myth showed in some examples from British Columbia, the calculation of the 85th percentile—and therefore the speed limit—could be way off.


Just A Matter Of Opinion, Too

Even so, there’s a body of research that suggests traffic fatalities go up when speed limits are increased—even as our shitty roads fall into total disrepair. Part of the reason, Wired points out, is that some places just want people to drive fast.

Western states are upping their limits anyway. Geography plays a role: Driving faster makes getting around rural areas more practical.

Regional politics matter, too. It’s no secret the states in questions are Republican territory (though a pair of researchers in Barcelona did a study to make sure). Upping the speed limit “sounds like such an easy regulatory win,” says Owen Gutfreund, a transportation historian at Hunter College. It’s a simple way to “get government out of your face,” , and “the consequences are remote and don’t come back to cause any problems.”

You can present all the traffic fatality data you want to the politicians voting on speed limit laws, Gutfreund says. But unless a speeding-related crash “has happened to them or someone close to them, it’s just a set of numbers on the page.”


There are places without speed limits—sections of Germany’s Autobahn, for example. That’s partly due to the design of those parts of the Autobahn, reports the BBC, and it maintains lower traffic-related fatality rates than speed-limited countries. though groups have long lobbied for some sort of limit to be implemented. (Still, some German groups would love to see some sort of cap put into place.)

All that aside, if you’re a visual learner, the website Seeker dove into speed limits in 2016, and produced a helpful 5 minute clip on the topic:

The U.S. Federal Highway Administration also has a super informational report to peruse—if you’re feeling 120 pages on speed limits. Otherwise, I hope I answered the question.