August 24, 2017

The National Transportation Safety Board (NTSB) recently released a report on speeding and passenger vehicle crashes. While NTSB has investigated speed as a contributing factor to specific crashes in the past, this is the first time that NTSB has investigated speed as it relates to crashes in general.

The focus on speed is long overdue – there were 112,580 speed-related deaths between 2005 and 2014. NTSB should be applauded for thinking about how to prevent speeding related crashes in the future.

The main conclusion of the report is that speed increases both the risk of a crash and the severity of injuries when crashes occur. NTSB issued 19 recommendations for decreasing the prevalence of speeding related injuries, including the following: increasing automated enforcement, improving speeding related data collection, increasing the availability of intelligent speed adaptation on new vehicles, reconsidering the 85^th percentile rule of thumb, and increasing the use of the safe systems approach to design in urban areas.

The NTSB study notes the geometric design of the roadway, traffic flow, roadside development, and the posted speed limit influence speed. Frequently there is a mismatch between the design speed, the posted speed, and the observed speed on a roadway. The reason for this is that, after an engineering speed study, the speed limits on roadways may be changed to match the 85^th percentile observed speed on the road. NTSB’s report notes that this “85^th percentile rule” may not improve safety. The rule is based on good research and sound reasoning, as major differences in vehicle speeds result in high risk of a crash and the likelihood of an injury.

However, it fails to account for vulnerable road users, and ignores the fact that increasing speed itself increases the risk of crash and injury. Rather than changing the posted speed limit to match driver behavior, engineers should use the safe systems approach to design roads that encourage safe speeds.

The safe systems approach, the motivating philosophy behind Vision Zero, is not only about preventing deaths to vulnerable road users (although roads that are safer for vulnerable road users tend to be safer for everyone). Rather, the “safe systems” approach acknowledges that fast moving motor vehicle traffic is inherently unsafe, and that human beings will make mistakes.

Transportation design in America relies on similar principles, but in-person enforcement and resetting speed limits tend to be emphasized – enabling drivers to make errors rather than preventing them in the first place. By designing roads to discourage speeding and implementing automated enforcement, we can reduce excessive speeding – thereby reducing injuries and deaths.

As noted in the NTSB report, many cities are rethinking how to design roads to increase safety. However, the emphasis on speed and design must not be limited to major cities; road deaths are as much of a rural issue as an urban issue.

In fact, the first major Vision Zero initiative in Sweden focused on rural roads. And while just 19% of the US population lives in rural areas, 49% of road deaths occurred on rural roads. Further, the fatality rate per 100 million vehicle miles traveled is 2.5 times higher on rural roads than urban roads. As the NTSB study notes, more deaths occur on local roads in rural areas than in urban areas. Local roads are the lowest on the US road hierarchy, where speeds should be lowest.

By thinking about how to reduce the speed of vehicles, transportation professionals are beginning to think about transportation safety as a public health issue, and for good reason. Traffic injuries are the leading cause of death amongst young people aged 16-24 in the United States, comparable to diabetes and suicide in terms of years of life lost.

By identifying speed as a major risk factor and designing the transportation system to limit speed and differentials in speed, transportation professionals are applying population health principles to solve what is truly a population health issue.

Take, for example, the Health Impact Pyramid, as described by Dr. Thomas Frieden, former director of the U.S. Centers for Disease Control and Prevention (CDC). This pyramid prioritizes interventions that have increasing population impact and decreased individual effort. At the top are those interventions that require high amounts of individual effort, and have a relatively small population impact. The interventions offering the most value are at the base, indicating high population impact and low individual effort. By focusing on how to design for the appropriate speed, transportation professionals can change the context in order to make the default choice the healthy choice.

There has been remarkable progress in transportation safety in the past 50 years – increases in seat belt use and decreases in drunk driving have already saved countless lives. However, speed still has not received attention or effort equivalent to the risk it poses. By rethinking our system and designing a safer one with safer speeds, we could prevent tens of thousands of deaths each year.

The views expressed above are those of the author and do not necessarily reflect those of the Eno Center for Transportation.