How Are Speed Limits Set?

There’s no shortage of speed demons barreling down the highway (nor is there a lack of overly cautious drivers moving far slower than necessary). But typically, motorists hover around the speed limit, which may sit at 15 mph in slower zones and spike up to 85 mph on vast stretches of open road. 

Where do these numbers come from though? Are they suggested by a highly trained computer program? Debated in darkness by a secret cabal within the Department of Transportation? What makes 30 mph a safe speed on one road, but 45 mph on another?

There’s no definitive set of rules for choosing speed limits, but there is a logic to the numbers. Depending on where you’re driving, there are a few different ways to draw the line between what’s legal and what gets you stuck with a speeding ticket.

The 85th Percentile Rule

While a nationwide maximum speed limit of 55 mph was in effect for more than 20 years, the control over setting speed limits was handed back to the states in 1995. These rules of the road are now implemented by government agencies, including state departments of transportation as well as county and local legislatures.

The people who set speed limits aren’t picking slower speeds just to make you late for work. Blanket speed limits can result in traffic that is too slow or too fast, since no two roads are alike — not just in length, width, or shape, but also in the traffic patterns they carry.

Two metrics often considered when setting speed limits are the 50th percentile speed and the 85th percentile speed. These are determined after collecting 24-hour traffic data during a standard weekday period before limits are set. The 50th percentile represents the median speed of traffic on a given road, whereas the 85th percentile refers to a speed at which 85% of drivers travel below and 15% of drivers travel above.

This latter metric is often considered an appropriate speed to drive, meaning the 15% traveling faster on that particular road are moving at speeds more likely to cause accidents. If, say, 25% rather than 15% of motorists are constantly going over the limit, the speed limit may be too low. Typically, the limit is set in 5 mph increments near the 85th-percentile speed, with adjustments for other conditions.

The Limits of Speed Limits

There are, however, rare cases where speed limits don’t end in a five or zero. For instance, if you happen to be driving through Appleton, Wisconsin, you may come upon an unusual 17.3 mph speed limit. This is an uncommon — but intentional — choice by the Outagamie County Recycling and Solid Waste facility, which posted that limit for safety reasons: The odd-looking number is meant to catch the eye of speedy motorists, as are the 31 mph limit in Trenton, Tennessee and 26 mph limit in Alabama’s Gulf State Park

And of course, the 85th percentile speed guideline isn’t a hard rule. Additional factors are taken into account when determining speed limits, including the location or characteristics of the road itself (i.e., a four-lane highway in the city versus a winding road in a small town), the history of accidents on any given road, the average daily traffic congestion, and to a lesser degree, the effect that traveling at each speed will have on air and noise pollution.

Many jurisdictions also use a computer program called USLIMITS2 to determine speed limits. It’s kind of like Ask Jeeves, except the only question it can answer is “How fast should people be driving?” Engineers or local government officials can enter various data points about a road into the system, which then uses an algorithm to suggest a recommended speed limit. The algorithm is designed by a panel of experts using 50th and 85th percentile statistics, pedestrian activity, on-street parking, and other information.

One important thing to note is that speed limits note the maximum safest driving speed in favorable driving conditions. This means that it falls upon every motorist to reduce their speed when driving conditions are less favorable, such as poor visibility from fog or heavy rain.