Increasing speed limits will likely result in more road casualties according to research – image © courtesy of Mike Woof
New research in the US shows that lower traffic speeds cut crashes and help to improve road safety. While there are moves by many local transportation agencies to increase speed limits so as to help increase traffic flow, this could well result in worsening road safety statistics.
Many posted speed limits are already being increased on roadways across the US. But there is concern that the increase could reduce the ability of vehicles to protect occupants in the event of a crash.
But even incremental speed increases can have major impacts on crash safety. New crash tests were carried out jointly by the AAA Foundation for Traffic Safety, the Insurance Institute for Highway Safety (IIHS) and Humanetics. The three bodies conducted controlled crashes at three different impact speeds of 64, 80 and 90km/h. The results showed that even slightly higher speeds were sufficient to increase the risk of severe injury or death for vehicle occupants.
Drivers often travel faster than posted speed limits, but when officials raise limits to match travel speeds, people go faster still. Today, 41 states in the US allow speeds of 112km/h (70mph) or higher on some roadways. There are eight states that have maximum speeds of 128km/h (80mph) or more. A 2019 study by the Insurance Institute for Highway Safety (IIHS) found that higher speed limits have cost nearly 37,000 lives over 25 years. The AAA and IIHS are joining forces to highlight to policymakers the potential risks from higher speeds when considering changing speed limits.
“We conducted these crash tests to assess the effect of speeds on drivers and learned that a small increase could make a big difference on the harm to a human body,” said Dr David Yang, executive director of the AAA Foundation for Traffic Safety. A speeding driver may arrive at their destination a few minutes faster, but is the tradeoff of getting severely injured or even losing one’s life worth it if a crash occurs?
The AAA Foundation collaborated with IIHS and Humanetics, the leading manufacturer of biofidelic crash test dummies, 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 US roadways and earned the top rating in the IIHS moderate overlap front test. Calspan Corporation conducted all the tests in its crash laboratory in Buffalo, New York.
As the crash speed increased in the tests, researchers found more structural damage and greater forces on the dummy’s entire body.
Higher speed limits cancel out the benefits of vehicle safety improvements like airbags and improved structural designs, said Dr David Harkey, IIHS president. The faster a driver is going before a crash, the less likely it is that they’ll be able to get down to a survivable speed even if they have a chance to brake before impact.
At the 64km/h impact speed, there was minimal intrusion into the driver’s space. But at the 80km/h impact speed, there was noticeable deformation of the driver side door opening, dashboard and foot area. At 90km/h, 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.
“Our crash test dummies are instrumented with hundreds of sensors to measure the injury risk so that we understand the scientific limits of safety and injury prevention. Understanding that the risk of serious and permanent injury becomes significantly higher in crashes beyond statutory speed limits clearly demonstrates why there are limits in the first place,” commented Jack Jensen, vice president of engineering at Humanetics.
At both 80 and 90km/h, 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 maximise all public road users’ safety.
“Cars are safer than they’ve ever been, but nobody’s figured out how to make them defy the laws of physics,” said Harkey of IIHS. Both the AAA and IIHS agree that rather than raising speed limits, states should vigorously enforce the limits they have. This includes using proven countermeasures like high-visibility enforcement.
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.
“Policymakers need to also think beyond enforcement to control speeds and should consider infrastructure changes based on road type to calm traffic flow appropriately so that posted speed limits are followed,” said Jake Nelson, AAA director of traffic safety advocacy and research.
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.
The research tests were conducted following the same protocol that is used for the IIHS moderate overlap evaluation; only the speed was varied. With a test dummy representing an average-sized male in the driver’s seat, the cars were crashed with 40% of the vehicle’s front on the driver side overlapping the barrier.
IIHS has been conducting this type of test, which simulates a head-on, partial-overlap impact between two vehicles of the same weight and size traveling at the same speed, since 1995. Since 2013, 100% of new vehicles have earned a good rating when tested at the 64km/h impact speed.
