The Science of Speed Limits

Speed limits actually predate automobiles – 200 years before the first car, Newport, Rhode Island forbade horses from galloping on major streets to prevent pedestrian deaths. Additionally, Boston, Massachusetts, restricted horse-drawn carriages to "foot pace" on Sundays to protect people walking to and from church.

The first gasoline-powered automobile was introduced in 1885, and it could reach a maximum speed of 13 mph. Technology has made it possible for today's cars to go 10x faster than that, and it's estimated that speeding is a factor in as many as one-third of all fatal crashes. Speed limits are in place to keep everyone on the road as safe as possible, and most motorists accept them as a part of the driving experience.

But who decides how fast is too fast? And how are speed limits determined?

Speed Matters

The laws of physics determine that as speed increases, so does the severity of accidents, yet for years traffic safety experts have disagreed as to whether or not speed also increases crash frequency. This is because road design and driver behavior can muddle the true relationship between speed and accident rates. The most recent research, however, strongly indicates that with all other factors being equal, higher speeds lead to a higher crash occurrence.

Who Sets Speed Limits?

Historically, both state and local governments have been responsible for setting speed limits in the United States. There have been two exceptions to that authority – once during World War II, and again when the National Maximum Speed Limit was established at 55 mph. This was repealed in 1995.

However, this is no wide consensus on how exactly to set limits. Traffic engineers use their experience and judgment when determining speed limits, and they employ several different techniques.

Methods for Setting Speed Limits

Different regions of the world favor various methods for determining speed limits. The primary systems are:

Engineering approach, operating speed: A baseline speed limit is set by engineers according to the 85th percentile speed. The concept of 85th percentile speed is based on the theory that:

  • Most drivers are reasonable and use good judgment, don't want to be in an accident, and want to reach their destination in the shortest time possible.

The speed limit is then set at or below the speed at which 85% of people drive under good conditions. This is primarily a safety measure – data has shown that traveling at or near the mean speed of 85% of drivers results in the lowest crash risk. It also allows police officers to more easily enforce speed limits – if only 15% of drivers exceed the speed limit, officers can spend their resources citing the most serious offenders.

Used in: United States

Engineering approach, road risk: The speed limit is set based on the function of the road (urban, rural, etc.). It is then adjusted to account for road and traffic conditions and crash history.

Used in: Canada, New Zealand

Expert system approach: A computer program sets speed limits using knowledge and procedures that mimic the judgment of speed limit experts.

Used in: United States, Australia

Optimization: Speed limits are set to minimize the societal costs associated with driving (e.g., travel time, vehicle operating costs, accidents, and air pollution). This method assumes that individual drivers do not weigh the toll that their driving takes on society at large (fuel consumption, noise, emissions, etc.). The optimal speed for an individual driver may differ from the optimal speed for a community.

No widespread use in any jurisdiction

Injury minimization/safe system approach: The goal is to create speed limits that manage crash energy so that no driver is subject to impact forces that could cause death or serious injury. It takes the view that it's unethical to create speed limits that are likely to cause human injury in order to reduce fuel consumption or minimize travel times.

Used in: Sweden, Netherlands

Drivers Are in Control

Speed limits are set to let drivers know what an acceptable speed is during favorable driving conditions. It's expected that drivers will reduce their speed during hazardous conditions such as poor visibility or when pedestrians or cyclists are present. Like anything else related to driving – whether it be using a seatbelt, ignoring your cell phone, or calling a taxi after a few drinks – driver responsibility is key.

Speed limits are in place to keep people safe, but it's up to motorists to follow the law. Not everyone does, however, and sometimes accidents happen. If you, or someone you love, have been the victim of a car accident, don't hesitate to get in touch with us. We're available 24/7.


“Methods and Practices for Setting Speed Limits: An Informational Report.” U.S. Department of Transportation, Federal Highway Administration.

October 15, 2014.

“Ask the traffic engineer: How are speed limits set?” California Association of Bicycling Organizations. January 30, 2010.

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