Speed and Injury: Is Raising the Speed Limit Worth the Risk?

by Gwen Newell
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Drivers want to save time, and local transportation agencies want to improve traffic flow, but at what cost? With posted speed limits increasing on roadways around the country, a vehicle’s ability to protect drivers in crashes is in doubt.

Small speed increases can have huge effects on crash outcomes, as shown in new crash tests by the AAA Foundation for Traffic Safety, the Insurance Institute for Highway Safety (IHHS) and Humanetics. The safety organizations conducted crashes at three different impact speeds (40, 50, and 56 mph). They found slightly higher speeds were enough to increase the driver’s risk of severe injury or death. AAA – The Auto Club Group provided funding for the crash test research.

Drivers often travel faster than posted speed limits, but when officials raise limits to match travel speeds, people still go faster. Today, 41 states – including both North and South Carolina,  allow 70 mph or higher speeds on some roadways, including eight states that have maximum speeds of 80 mph or more. A 2019 IIHS study found that rising speed limits have cost nearly 37,000 lives over 25 years. AAA and IIHS urge policymakers to factor in this danger from higher speeds when considering speed limit changes.

The AAA Foundation collaborated with IIHS and Humanetics to examine how speed affects the likelihood and severity of occupant injury in a crash. Three 2010 Honda CR-V EX crossovers were used because they represented the average age (11.8 years) of a typical vehicle on U.S. roadways and earned the top rating in the IIHS moderate overlap front test. As the crash speed increased in the tests, researchers found more structural damage and greater forces on the dummy’s entire body.

At the 40 mph impact speed, there was minimal intrusion into the driver’s space. But at the 50 mph impact speed, there was noticeable deformation of the driver side door opening, dashboard and foot area. At 56 mph, the vehicle interior was significantly compromised, with the dummy’s sensors registering severe neck injuries and a likelihood of fractures to the long bones in the lower leg.

At both 50 and 56 mph, the steering wheel’s upward movement caused the dummy’s head to go through the deployed airbag. This caused the face to smash into the steering wheel. Measurements taken from the dummy showed a high risk of facial fractures and severe brain injury.

When correctly set and enforced, speed limits improve traffic flow and maximize all public road users’ safety. Speed limits should not be raised or lowered only to manipulate traffic volume on a particular roadway. States are urged to use engineering and traffic surveys when setting maximum speed limits.

This study is the second part of the AAA Foundation for Traffic Safety research examining the effect of posted speed limit changes on safety. In the Foundation’s first study, traffic engineers were asked how posted speed limits are set and what factors they consider in changing them.

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