Research Team: PI: Brian D. Taylor Team: Yu Hong Hwang

About this project:

This research synthesis will review the history and evolution of the 85th percentile rule in traffic engineering practice, and critically analyze and summarize research to date on its effects. The resulting report, policy brief, and Infographic will provide a complete and succinct summary of how the rule evolved into practice, and what researchers have found about its effects.

With respect to the logic of the 85th percentile rule, it rests on a series of interlocked assumptions that have been largely accepted as given by traffic engineers for decades:

1) That 15 percent of drivers will, absent speed limits, drive faster than is safe to do so over a given stretch of road.
2) That 85 percent of drivers will, absent speed limits, drive at or below safe speeds over a given stretch of road.
That #1 and #2 do not vary significantly from region to region.
That #1 and #2 do not vary significantly among cities, suburbs, and rural areas.
That #1 and #2 do not vary significantly with the mix of drivers in the traffic stream (with respect to age, gender, familiarity with the road, and so on)
That #1 and #2 have not changed significantly over time.
That #1 and #2 have not changed in light of efforts to create more “complete” streets that host social and economic activity on sidewalks and in parklets, more bikes and scooters in the roadway, and more pedestrians crossing trafficways.
That what an 85th percentile driver feels is an optimal travel speed (balancing personal utility and risk) is actually optimally safe for occupants (both those in and outside of vehicles) of a given roadway segment.

This is a very large set of assumptions, some of which have been studied, but most of which have received little consideration from traffic engineers applying this widely accepted universal standard for setting speed limits. This review will consider these assumptions in its research review in order to make recommendations about the appropriate role of the 85th percentile rule in light of changing priorities for and uses of streets and roads. The review will also consider alternative methods for setting speed limits and will consider their advantages and disadvantages with respect to both traffic flow and safety vis-a-vis the 85th percentile rule.

What problem does this research aim to address?

Travel in California is dominated by driving, which brings many travelers great benefit, but at very high cost to the environment and society — ubiquitous sprawling development, chronic traffic congestion, high levels of pollutant and greenhouse gas emissions, and a heavy toll of traffic-related deaths and injuries. Over 3,500 people have died on California’s streets and highways each year since 2016, despite commitments at the state, regional, and local levels to reduce this toll.

While much progress has been made in making motor vehicles safer, and efforts to reduce driving while intoxicated have had success as well, a growing number of safety experts have pointed to high speed limits as serious obstacle to increased traffic safety. Safety research has long demonstrated a correlation between speed and the severity of crashes, but as policies that aim to promote alternatives to driving put more pedestrians, cyclists, scooterers, and people walking to and from public transit in the path of motorists, the logic of how speed limits are set is being called into question.

The basic rule for setting motor vehicle speed limits in California, and across the U.S. is the so-called “85th Percentile Rule,” which involves measuring a sample of speeds on a given roadway link and setting the speed limit below the speed traveled by the 15 percent of the fastest speeds measure, and above the 84 percent of the slowest speeds measured. This rule is deeply ingrained, both practically and legally in transportation engineering practice, but is now being scrutinized by those committed to improving traffic safety.

What are the expected impacts and benefits of the research?

This research synthesis will be presented in three products intended to inform debates over traffic safety and speed limit-setting in California. A final report approximately 10,000 words in length; an ITS Policy Brief approximately 500 words in length, and an Infographic to provide decision-makers, journalists, and others with a quick to understand summary of the principal findings and recommendations of the report.

By |October 3, 2019|Categories: Traffic, Transportation & Communities