IIHS Study Finds That Increases in Highway Speed Limits Lead to Higher Number of Fatalities
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IIHS Study Finds That Increases in Highway Speed Limits Lead to Higher Number of Fatalities

April 19, 2016

A data analysis by the Insurance Institute for Highway Safety (IIHS) found that increases in speed limits in the past 20 years have also increased the number of auto-related deaths. Overall, auto fatalities have decreased, but IIHS researchers assert that the numbers would have been lower if speed limits had not increased during the same period cars became safer.

According to the study, increases in speed limits over two decades have cost 33,000 lives and in 2013 alone, the increases resulted in 1,900 additional deaths, cancelling out the number of lives saved by airbags.

"Although fatality rates fell during the study period, they would have been much lower if not for states' decisions to raise speed limits," study author Charles Farmer said in a statement.

During the energy crisis back in the 1970s, in order to get their share of highway funds, states had to adopt the National Maximum Speed Limit of 55 mph. The IIHS said this move resulted in a decrease in auto fatalities.

The law, however, was fairly short lived. In the late 1980s, states were allowed to increase speed limits to 65 mph on rural interstates. Less than 10 years later, in 1995, the law was completely repealed. According to the IIHS, six states have 80 mph limits and drivers in Texas can speed along at 85 mph on some roads.

Previous studies done by the IIHS found that as travel speeds increased, so did fatalities, first on rural interstates and then on all interstates.

Study Methodology

Farmer looked at all speed limit increases from 1993 to 2013 in 41 states with the most consistent information. Looking deaths per billion miles traveled and roadway type, Farmer found that each 5 mph increase in the maximum speed limit resulted in a 4 percent increase in fatalities. The increase on interstates and freeways was 8 percent. Farmer also took into account other factors that can affect fatality rate including changes in unemployment, the number of potential young drivers (ages 16 to 24) and per capita alcohol consumption.

Farmer then compared the annual number of fatalities with the number that would have been expected if each state's maximum speed limit had remained unchanged since 1993. He found an estimated 33,000 additional fatalities during the 20-year period.

Farmer believes that the number is an underestimate since he only considered increases in the maximum speed limit.

Since 2013, five more states have increased their maximum speed to more than 75 mph and other increased their max from 65 to 70 mph.