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Using Adaptive Cruise Control Leads Drivers To Speed, Says IIHS

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April 7, 2021Drivers are substantially more likely to speed when using adaptive cruise control (ACC) or partial automation that combines that feature with lane centering than when not using either technology, an Insurance Institute for Highway Safety study showed. When selecting a speed to “set and forget,” many drivers choose one that’s over the limit.

“ACC does have some safety benefits, but it’s important to consider how drivers might cancel out these benefits by misusing the system,” said IIHS Statistician Sam Monfort, the lead author of the paper. “Speed at impact is among the most important factors in whether or not a crash turns out to be fatal.”

ACC is a more advanced version of traditional cruise control that uses sensors to calculate and maintain a preselected following distance from the vehicle ahead, eliminating the need for the driver to repeatedly brake and reset the system. With the addition of lane centering, the vehicle also maintains its position within the travel lane automatically.

The systems on the market today don’t restrict drivers from setting speeds that are higher than the legal limit, and they require constant supervision by the driver because they’re not capable of handling certain common road features and driving scenarios.

To find out the impact ACC and lane centering technologies have on speeding, IIHS researchers analyzed the behavior of 40 drivers from the Boston metro area over a four-week period using data collected by the Massachusetts Institute of Technology’s Advanced Vehicle Technology Consortium. The data suggest that drivers were 24 percent more likely to drive over the speed limit on limited-access highways when those systems were turned on. The amount by which they exceeded the speed limit when they did speed was also greater when they were using the driver assistance features compared with driving manually.

Whether driving manually or using ACC or Pilot Assist, speeders exceeded the limit by the largest margin in zones with a 55 mph limit. In these areas, speeders averaged about 8 mph over the limit, compared with 5 mph in 60 mph and 65 mph zones. ACC also had the largest impact on how much they exceeded the limit in zones where it was 55 mph. In these slower zones, they averaged a little more than 1 mph higher over the limit when using ACC or Pilot Assist than they did driving manually.

That 1 mph increase may not sound like much, but it means that ACC and partial automation users are at about 10 percent higher risk of a fatal crash, according to a common formula for calculating probable crash outcomes utilized by IIHS. 

To be clear, ACC is intended to lower crash risk, and studies have shown that these systems maintain a greater following distance than most human drivers, change lanes less frequently, and respond sooner and less abruptly than human drivers. But if these systems aren't repaired or calibrated correctly and drivers are speeding while using them, it could result in a more severe collision, for which the shop would be liable.

IIHS suggested that future research will need to balance these benefits against the effects of excess speeding to fully understand the technology’s impact on safety. Making systems more restrictive might be the answer, provided that limiting the maximum speed or linking it to posted limits doesn’t discourage risky drivers from using ACC altogether.

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