Farmer looked at annual traffic fatalities per mile traveled for each state, and his math shows that a 5-mph increase in the maximum speed limit was associated with an 8 percent increase in the fatality rate on highways.
This theory involves more than just transitive mathematical equations. IIHS also adjusted for the unemployment rate, the proportion of young drivers in the population, and seat belt usage rates.
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Obviously, each 5-mph increase in speed also increases the distance a vehicle travels from the time a driver detects an emergency to the time a driver reacts. It increases the distance needed to stop a vehicle once the driver starts to brake. And every mph increases the crash energy exponentially. For example, when the impact speed increases from 40 to 60 mph (a 50 percent increase), the energy that needs to be managed by the car’s crash structure increases by 125 percent, according to IIHS.
Although the study did not break out fatality rates by individual state, on average, high-speed-limit states such as Texas, Montana, Utah, Nevada, and Wyoming suffered higher frequency of fatalities.
I asked Farmer if having more SUVs on the roads contributed to the numbers—given their relative inability (compared to sedans) to avoid accidents due to the facts that their higher center of gravity affects handling and their added bulk increases braking distances. There’s also the theory, posited by Tom Vanderbilt in his landmark book Traffic, that by sitting higher, SUV drivers have a lessened ability to sense the road’s “textural density” and therefore drive faster than conditions warrant and are also slower to react to potentially dangerous situations.
“Newer-model SUVs have some of the lowest fatal crash involvement rates,” Farmer said. “We have not seen evidence that they get into more crashes than other vehicle types.” So much for that idea (IIHS-tested vehicles are shown below).
My final haymaker at this report: I posited that people have a tendency to drive at their personal comfort level, regardless of speed limit. But there’s always the left-lane law-abider tootling along at the speed limit. If the speed limit is 65, that creates a hazardous pass-on-the-right situation for impatient drivers who feel road conditions warrant driving faster. But if the speed limit is 80, most folks are pretty comfortable at that speed and would likely allow the Prius to set the pace, lane discipline would be maintained, and safety might actually increase. True?
IIHS has an answer there, as well, from a 2016 study: “Raising speed limits results in higher travel speeds and more vehicles exceeding the new limit. It also undercuts the claim that raising limits reduces speed differences among vehicles on the same road.”
This is only the start of the conversation. Does the condition of our crumbling roads have an influence? With America’s aging vehicle fleet, how many of the fatalities occurred in older vehicles, rather than newer ones with better safety equipment? How do our fatality numbers compare to European countries where higher speed limits are commonplace? Food for thought.
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