Strict Speed Limits – Ignoring the 85th Percentile
Guest Post by Bryant Ficek, PE, PTOE, Vice President at Spack Consulting.
We’ve previously examined the 85th percentile speed and how it is used in Minnesota and many other places to set the speed limits on roads. Essentially, we take a survey of existing speeds and set the road speed limit at the point 85 percent of drivers will drive at or below under free-flowing conditions. This method has many advantages including better compliance by the drivers without needing excessive enforcement. However, there are other ways to go about setting speed limits.
In Australia, two different “default” speed limits are used. These automatically apply everywhere unless overruled by a posted speed limit sign. The defaults are:
- 50 km/h (31 mph) within built-up or urban areas
- 100 km/h (62 mph) outside built-up areas or rural areas
In school areas, the posted speed is usually reduced further to 40 km/h (25 mph). In other pedestrian heavy areas, the speed limit is often posted at 10 km/h (6 mph).
These laws have been set based on a desire to improve safety for all users. Research has shown that:
- A crash is less likely to occur speeds of 50 km/h (31 mph) as drivers have more reaction time
- Pedestrians are significantly more likely to survive a crash at speeds below 50 km/h (31 mph)
These limits were introduced in 2001 on roads in the Australian state of Victoria and their research has shown fewer deaths since that time even as vehicle miles traveled has increased.
Ideally, all the roads would be designed with these speeds in mind meaning the desired 85th percentile speed of drivers would match the speed limits. In reality, Australia relies on heavy enforcement to compel drivers to obey the limits. Speed cameras, like red-light running cameras, are located along many corridors and do not require a police officer’s time to fine and put demerit points on a license. Radar and LIDAR are also widely used in addition to aircraft in some areas. The ability to have fixed and mobile camera sites in particular has the desired effect on slowing drivers to their speed. Engineering also gets involved where they can in the form of signal timing on corridors.
There are pros and cons to this type of approach, including privacy concerns with cameras (as we and others have learned in attempting to implement red-light running cameras). I don’t think Minnesota or the U.S. in general will go down this road. It was tried briefly in the 1970’s with a national speed limit of 55 mph on the interstates and quickly went away in the face of public opposition.
A better approach might be to decide the desired speed limit for a roadway based on function, land use, and other characteristics. Then during construction or reconstruction, add engineering elements to naturally enforce that design. This could include roundabouts at intersections, introducing horizontal or vertical curvature, using landscaping to limit sight lines, or many other measures under our control during this time. When a driver instinctively uses these characteristics, they won’t know we’ve done our job to reduce speeding without police enforcement.