How Do Traffic Engineers Decide Speed Limits?

There are hundreds of millions of motor vehicles on U.S. roads, and tens of millions of people sharing them as drivers, pedestrians, bicyclists, skateboarders, and electric scooter users. It falls on traffic engineers to decide what speeds are appropriate for vehicles to negotiate a given roadway. This can be a really complicated job that sits at the intersection of science, civil service, and policy.

States determine their own speed limits, of which there are two types. Statutory speed limits are uniform and typically apply to common roads throughout a given jurisdiction. For example, Oregon limits vehicles in residential areas to 25 mph, and passenger vehicles are limited to 65 mph on interstates. Speed zones – which have come under recent scrutiny – are where statutory speeds merge with context, making a lower or higher speed limit more appropriate based on engineering studies.

Traffic engineers usually use one of three methods to decide speed limits: Engineering, Expert System, and Safe System approaches. Optimization is a fourth approach that's not widely used on its own, but it adds context for community concerns like air pollution or noise. The Engineering and Expert System approaches are most widely used in North America, and they both rely on the observed 85th percentile speed, which focuses on how fast 85% of drivers actually go when traffic is flowing smoothly.

In the simplest terms, the Engineering approach uses one of two paths to decide a speed limit. The Operating Speed Method starts with a base speed limit, determined either by the road's design (say, straight or curved) or the 85th percentile speed. Traffic engineers then set a speed limit within five mph of that base before adjusting for speed surveys, crash data, pedestrian use, and so on. In some cases, engineers might instead use the Road Risk Method, which prioritizes the road's function and development status to set the base speed limit. 

An Expert System — such as the Federal Highway Administration's (FHWA) USLIMITS2 knowledge base — uses inputs from traffic engineers and road use data to algorithmically suggest speed limits. Computers factor in everything from 50th and 85th percentile operating speeds to traffic flow and area development, as well as parking, crash rates, pedestrian use, and injury data. After the data is crunched, recommendations are made in increments of five mph relative to operating speeds. This method could be considered part of a larger push for technology and data to help shorten commute times and slash red light delays.   

Ditching the 85th percentile standard has also gathered steam, but state laws and red tape can make it hard for local jurisdictions to make changes. The National Association of City Transportation Officials (NACTO) is part of a movement to reconsider how traffic engineers fundamentally decide speed limits by taking a pedestrian's perspective and rethinking risk.

Speed zones are at the crux of the call for traffic engineers to adopt new methodologies to emphasize context in setting speed limits — specifically with higher regard for "non-motorized road users, such as pedestrians, bicyclists, other cyclists, and persons on personal conveyances, who cannot rely on vehicle bodies or technologies to protect them," as the FHWA explains in its Speed Limit Setting Handbook (2025). It goes on to say "the 85th-percentile speed should not be used as the sole consideration in setting speed limits." In other words, particularly in densely-populated areas, base speeds shouldn't necessarily or uniformly be calibrated by how fast drivers want to go.

There's a focus on helping states address where higher speeds and injury severity correlate. Between 2009 and 2023, motor vehicle-pedestrian crashes increased 78%, according to the Insurance Institute for Highway Safety. The FHWA spotlight on safety and context echoes NACTO's 2020 City Limits publication, which offered traffic engineers new tools to weigh street activity and decide urban speed limits: "The risk matrix is based on the idea that high conflict, high activity streets will require lower speed limits since the risk of a crash is high, while low conflict, low activity streets can tolerate somewhat higher speed limits."

This hearkens to the Safe System method used by Sweden and the Netherlands. It sets speed limits based on the types of crashes likely to occur on a given roadway, and how those forces affect pedestrians. While no system is perfect, and speed is one part of the puzzle, these potential changes will hopefully lead to safer roadways for everyone.