Speed Limits - A Deeper Dive

We mentioned the 85th percentile rule and USLIMIT2 in our last blog post, but what do those things really mean for our speed limits? These are both ways that engineers determine a proper speed limit for an area. The 85th percentile is when the speed of a road is set at 85% of the average driving speed, determined after a speed test. USLIMIT2 is a web-based program that also uses a speed test, but will also calculate the best speed limit for an area after considering other factors such as traffic, the 50th percentile, crashes and accidents, the amount of development in the area, and more. What are the pros and cons of both?

For the 85th percentile, the speed limit is set at slightly below the top speed that the average driver travels. This accounts for the tendency drivers have to speed, but doesn’t always mean that it’s the safest speed for the road. If there’s an area where people tend to speed, the 85th percentile might set the limit to what is actually the ideal versus slightly below. This means that speeders will drive at exponentially more dangerous speeds. See the image below for how visual representation of how speeding only increases as speed limits increase, suggesting slightly lower speed limits could make for safer travel. The problem with the 85th percentile rule is that it assumes that drivers are traveling at reasonable speeds during the speed test, whereas usually, people will speed up to match the flow of traffic and once more cars are traveling at faster speeds, slowly most of the drivers will increase in speed to match. When a higher speed limit is posted to match the average driving speed, or even to equal the 85% of the average driver, people still chose to speed an average of 5-15mph over the posted speed limit. Instead of increasing the speed limit to match drivers, we should take other things into account and if there’s a tendency in the area for people to speed regardless, set the limit slightly below the 85th percentile.

USLIMIT2 does consider these additional factors and can be a very efficient tool for determining speed limits. However, it also has its limitations. The program cannot determine speed limits for things like school zones, construction zones, or roads with varying speed limits due to weather or traffic conditions. This is when engineers step in and use their own expertise to determine what the best speed limit should be, according to their own information.

To be honest, both seem to be pretty okay at determining what our speed limits should be. Both require prior information and take statutory limits and human tendencies into account. The con with both is needing to modify for unpredictable factors like human rebellion and sudden dangers like weather and traffic conditions. The best option remains to combine a web-based program with human experience and skill for the safest and most efficient speed limits in an area. Now, when you pass a speed limit sign you can know how that speed was determined, and remember, we made that sign that kept you safe on your way!