Historically, traffic engineers have used the 85th Percentile Rule method to establish the speed limits posted on our roads and streets; but for the context of the environment, we should be using other design features to determine our speed limits.
The 85th Percentile Rule method was developed in the 1960’s as a metric designed for use on rural roads that was then applied to all types of roads, highways, residential and city streets. When this type of method, established for the design of rural roads, is applied to downtown roads, city streets, or more complex street networks that include multiple access points, buildings or houses close to the roads, and include other street users such as bicyclists, pedestrians, and public transit, these 85th percentile speed limits often conflict with the overall intended use of these types of streets.
The 85th Percentile Rule method tends to involve drivers driving a speed on the road that is subconsciously “comfortable” to them or driving at a speed that “feels” safe. This is most often related to the design speed of the road that includes features such as lane widths, straight roadway sections, gradual curves, lack of curbs and sidewalks, wide shoulders, and wide off-road clear zones and shoulders. New roads are often designed for speeds higher than what is posted which is in conflict to the cues the design of the road is providing the driver vs. the posted speed limit. Often these speed limits are assigned by local governments based on local ordinances or political concerns rather than traffic engineering data. To add to the confusing and conflicting information for drivers, many states and cities establish maximum speed limits or have ordinances written that limit the ability for these speed limits to be lawfully enforced within certain speed limit thresholds.
It is important to understand that the design of the road gives the drivers a clear sense of the speed the drivers should be driving, with or without a posted speed limit. A roadway with large straight sections, wide lanes, and with sidewalks and buildings set far off the road indicates to drivers that they can drive faster to their comfort level. But often these types of roads have a much lower posted speed limit that does not match the design speed of the road. This is even more evident in small rural towns where the posted speed limits are often tapered down in 10 mph increments entering the City Limits and through the town, but the road geometry and the design of the road itself is no different through the city. The speed limit signs are changed but the road design itself is not providing information to the driver to slow down in this area.
The design of the road indicates to the driver the speeds at which they “feel” safe driving. For these speeds to be lowered by design context, the design must provide features that make drivers feel as if they need to slow down or drive more carefully. There are many cities and towns that incorporate these types of design features to help lower the speeds on their roadways. Some of these features include more narrow lanes and streets, more horizontal curvature features, textured pavement, medians and bulbed curb-outs and bollards, vertical crosswalks, and using landscape features in medians and along streets between sidewalks to create a roadway that feels narrower and more condensed. These types of roadways also integrate other roadway user features for bicycles, pedestrians, and public transit uses. These design features can indicate to the drivers the speed that it “feels” right to drive on these types of streets and the context of the environment.
The old traffic engineering approach was to design the street, check the 85th percentile speeds and then set the speed limit. The safer approach should be to determine the speed limit that is safest for the use and context of the street being designed and provide those design features which convey that speed operation to the driver. If drivers are traveling above that expected speed limit, the design features can be addressed to obtain that desired speed. It’s important that the design of our roads start to match the context, use and purpose of the street environment in conjunction with the design speed and posted speed limit on our roads and streets.
About Stevie Berryman, PE
Stevie Berryman, PE is a Project Manager for Foresite Group’s Traffic Engineering Division in Peachtree Corners, GA. Stevie graduated from the Georgia Institute of Technology with a Master’s degree in Construction Management. In his spare time, Stevie is a coach for a youth travel baseball program.