The Insurance Institute For Highway Safety just released a report detailing the effects of higher speed limits on traffic fatalities between 1993 and 2013. Their conclusion: lots of people’s lives could have been spared if speed limits hadn’t risen.
The Insurance Institute For Highway Safety does a lot of data-crunching and statistical analysis on car accidents. Their latest study examines the results of repealing the National Maximum Speed Limit, a 55 mph limit that congress imposed on states starting in the 1970s.
That 55 mph limit—catalyzed by a fuel shortage, and imposed upon states by threatening to withhold federal highway funds—dominated the American highway system throughout the 1970s and 80s. Then, in 1987 the limit went up to 65 on rural highways, and in 1995, it was removed entirely, leaving states to decide on highway speed restrictions.
In 1999, IIHS released a study showing that after the 1987 speed limit increase, highway fatalities increased on rural roads. That paper also showed that after the federal limit was completely repealed in 1995, deaths on all highways increased.
IIHS’s latest study is a follow-up on that 1999 paper, looking at the effects of speed limit increases throughout a 20-year period from 1993 to 2013. IIHS’s research includes data from the 41 states with the highest number of vehicle miles traveled each year (IIHS says the remaining nine were omitted due to their high fluctuations in yearly fatality rates).
Charles Farmer, author of the research paper, analyzed deaths per billion miles traveled for each state and type of roadway, accounting for other factors that could affect fatality rates, such as the health of the economy, the number of young drivers, and per capital alcohol consumption.
After excluding those external factors, Farmer determined that each 5 mph increased in the maximum speed limit resulted in a four percent increase in overall vehicle fatalities.
Farmer looked at annual deaths in those 41 states and compared the values to those that his model thinks would have occurred had the speed limits not changed since 1993. He arrived at the conclusion that 33,000 additional fatalities occurred over the 20-year period as a result of increased speed-limits.
Farmer says he thinks that’s a conservative number, too, as his research only looked into increases in maximum speed limits (which are usually found on rural highways), and doesn’t consider increases in speed limits on urban highways.
Also, because the study stops in 2013, it doesn’t account for the five states that have since upped their limits from 75 to 80, or for the states that have ditched the 65 mph limit for 70 mph.
The good news is that overall vehicle fatalities have dropped during the time period in question, but Farmer’s point is that it could have dropped further. He said:
Although fatality rates fell during the study period, they would have been much lower if not for states’ decisions to raise speed limits.
It’s an interesting study, and one that shows that highway speed limits aren’t just annoying white signs keeping you from getting places quickly, they actually have real, measurable safety implications.