From the days of horse and carriage, speed limit science (speed science) has been used to help establish speed limits on how fast (or slow) people and their vehicles should go. I’ve spent years writing about automotive safety and maintenance, and I’ve seen how misunderstood and confusing speed limits can be.

Speed limits are public trust rules (or laws) intended to nurture road safety, traffic efficiency, and personal accountability. With the arrival of the automobile in the early 20th century, speed science has evolved into a highly specialized engineering field. Using copious data and logic, speed engineers (scientists) help plan and design roadways and infrastructure to meet changing environmental needs, vehicle technology, and driver behaviors. This creates a balance that allows for the proper flow of traffic while keeping drivers, pedestrians, and cyclists safe.

Ahead, experts in transportation engineering, defensive driving and the law will explain how speed limits are set, why the current system is under scrutiny, and what the future might look like.

How Are Speed Limits Set?

Traffic on a bridge with a speed limit of 50 and radar enforced
PorqueNoStudios/Getty Images

According to Nathan Kautz, transportation engineer at Kittelson and Associates, Inc., “Speed limits in the U.S. are generally set using the guidance provided in the Federal Highway Administration (FHWA) Manual of Uniform Traffic Control Devices (MUTCD) 11th Edition” MUTCD standards differentiate between urban, suburban, and developed rural arterials versus freeways, expressways, and rural highways, “as well as offer guidance on how to apply the 85th percentile and six factors that should be considered when setting or reviewing speed limits”, including:

  • Roadway environment
  • Roadway characteristics
  • Geographic Context
  • Crash experience
  • Speed distribution of free-flowing vehicles
  • Speed trends

“Unfortunately, most speed limits are reviewed reactively as a result of roadway changes, policy adjustments, zoning updates, or crash statistics,” added Robert Dillman, an authority on defensive driving.

Kautz also noted that although the MUTCD sets federal guidance for all traffic control, some municipalities use that as a basis for developing their own traffic control standards.

Are the rules the same around the US?

No. Mike Agruss, CEO and founder of Mike Agruss Law, acknowledged that “Speed limit rules and laws are very different all around the U.S.; varying not just from state to state but also between cities in the same state.”

Although there are federal guidelines set by the FHWA, states have the authority to set speed limits and revise them if needed. Based on crash statistics and other data, “if problems continue to arise on a specific road, it will likely cause a municipality to review the current speed limits or replace traditional and often dangerous four-way intersections with roundabouts, rotaries, or diverging diamond intersections,” said Dillman.

The 85th Percentile Rule Controversy

Kautz explained, “The 85th Percentile Rule is seen as the ‘traditional’ method of evaluating and setting speed limits.” Based on human behavior, rather than data or statistical analysis, “this approach to setting speed limits is that the majority of drivers will naturally select the safest speed to travel under normal conditions for that roadway,” said Wes Guckert, Chairman and CEO at The Traffic Group. Agruss added that the controversy “is based on the fact that roughly 85% of drivers will naturally choose the right speed as they drive on a particular road.”

Unfortunately, this method assumes drivers will actually follow the posted speed limit. The FHWA warned that “relying on driver behavior alone ‘may perpetuate high-speed travel’ at the expense of everyone else on the road.” FHWA published its newest edition of the MUTCD, it showed a shift away from the 85 Percent Rule, maintaining the rule was more of a suggestion than a rule.

The latest FHWA Speed Limit Setting Handbook stated, “Depending on the roadway context and conditions, setting speed limits based on the 85th-percentile may not represent an appropriate speed considering all road users and may perpetuate high-speed travel for the majority of drivers.”

“Developed in the mid-20th century when roads were simpler and traffic volumes lower, today, many argue the 85th Percentile Rule is outdated,” said Dillman. In addition, distracted driving, larger vehicles, and older road designs could put more vehicles, pedestrians and cyclists in danger. Proponents believe “formal assessments conducted by traffic engineers who measure the average speeds of vehicles traveling on the road, traffic volume, conditions of the road, land use, and crash data are needed,” said Kautz.

These inclusive assessments can help determine realistic speed limits for individual highways and byways, not just modern driver behavior or road design, and address some of the changes, such as “semi-autonomous vehicles and other technological advancements that did not exist when this rule was conceived,” said Dillman.

What Is Speed Study?

According to Guckert, “A speed study examines how fast vehicles are traveling in each direction [on a road or highway], typically by vehicle type over 24 to 48 hours to determine average vehicle and 85th percentile speeds.” Collected data are then compared to current posted speed limit data and “can show if there are issues and make recommendations [to authorities] as to whether to raise, lower, or keep the current speed limit,” said Agruss.

If, for example, the current posted speed limit is 35 mph, and 85 percent of vehicles are traveling at 40 mph or 45 mph, increasing the posted speed limit may be considered. Guckert adds that context and surroundings matter, and in all situations, “Engineering judgment comes into play. If the 35 miles per hour limit is in a school zone, the governing body typically decides to keep that speed limit or even reduce the speed limit because of the surrounding environment.”

How Often Are Speed Limits Reviewed?

“It depends,” said Kautz. Speed studies are difficult, time-consuming and expensive. He added, “Generally, speed limits are reviewed after a road or other infrastructure project has been completed.” The MUTCD suggests that speed studies should be done when a roadway has “undergone significant changes since the last review.”

In addition, citizens or municipal officials will request a speed limit review if the number of pedestrians and cyclists now using the roadway grows, or the number of crashes, especially fatal ones, increases. Technological advancements in cars and road building materials, how traffic impacts communities and the environment, as well as safety, have city planners and transportation engineers rethinking highway, intersection and on-off ramps designs. Designs that not only encourage drivers to travel at appropriate speeds but also help reduce bottlenecks while lowering travel times.

FAQ

Why not just set lower speed limits for safety?

Going too slowly is also problematic because it creates more bottlenecks in traffic, leading to driver frustration, uneven traffic flow, and noncompliance,” said Agruss. And compliance is the bigger issue. Traveling the PA Turnpike 90 minutes each day for 25 years, I found most vehicles/drivers consistently driving above the posted speed limit, during my daily commute, forcing me to also drive above the speed limit just to keep up with traffic. “The only locations where lower speed limits are routinely followed are school zones, work zones, or other high pedestrian traffic areas,” said Dillman.

“The best way to police speed limits is to install speed cameras,” added Guckert. “Once you receive several tickets, you will start to obey the speed limit.”

Are there minimum speed limits?

Yes. Highways usually post minimum speed limits to prevent slower-moving vehicles from triggering safety issues, such as rear-end collisions or disrupting traffic flow, that result in traffic backups. “Safe speed limits balance safety with traffic efficiency,” said Agruss.

About the Experts

Nathan Kautz is a transportation professional at Kittelson and Associates, Inc. He has 15 years’ experience working in transportation engineering and has been managing and performing speed studies both at a state agency, and as a consultant. He has been leading a Vision Zero Plan with the Polk Transportation Planning Organization and producing a statewide Safety Academy for the Florida Department of Transportation.

Mike Agruss holds a Bachelor of Arts in Law and Society from USC, Juris Doctor from University of Illinois (UIC), is a member of UIC John Marshall Journal of Information Technology, and is CEO and founder of Mike Agruss Law. One of Mike’s areas of concentration is road safety and how to reduce the number of vehicle accidents, as well as having a full understanding of transportation and speed limits laws.

Robert Dillman graduated from Arizona State University with a Master of Arts in International Security. He opened NEVO Defensive Driving Academy in 2012. He closed NEVO to work as a Security Driver Training Instructor for the Department of State.

Wes Guckert is a certified Professional Transportation Planner (PTP), a Harvard University Lecturer, as well as a Fellow with the Institute of Transportation Engineers and is Immediate Past Chair of Urban Land Institute’s Public Development and Infrastructure Council. Wes holds a Bachelor of Science degree from the University of the State of New York, is an expert in traffic engineering and transportation planning. He is a technical adviser in transit planning, traffic impact analysis, traffic signal design, traffic circulation, and access studies. He has played a major role in more than 9,000 projects in urban and suburban areas. He also holds affiliations with the American Planning Association (APA); Institute of Transportation Engineers (ITE); Transportation Research Board (TRB) and the Urban Land Institute (ULI).

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Bob Lacivita
Since rebuilding his first engine at age 14, cars have been Bob Lacivita's passion, as well as his way of life. He went on to become an award-winning ASE and General Motors auto technician, vocational educator, career and technical center administrator, technical consultant and freelance writer; teaching legions of students and readers about DIY ca...
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