Drivers who use the adaptive cruise control (ACC) function in their vehicles are more likely to set their vehicles to travel above the posted speed limit, according to a new study by the Insurance Institute for Highway Safety.

This has been found to be more likely to happen when drivers use ACC or partial automation in conjunction with lane centring (lane keeping assist) functions, as compared to doing without either kind of driver assistance, and when selecting a speed for the ACC function, drivers are more likely to select one that is over the posted limit, the report said.

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

Systems that are available in vehicles for sale today do not restrict drivers from setting cruising speeds that are above the posted speed limit, and the systems still require constant supervision by the driver because they are not capable of handling certain road features and driving scenarios, said the report.

That being said, a separate analysis of insurance claims data by IIHS-affiliated Highway Loss Data Institute (HLDI) indicated that adaptive cruise control actually serves to lower the risk of crashing.

The IIHS report also states that “other studies have shown that these systems maintain a greater following distance at their default settings than most human drivers, and suggested that they reduce the the frequency of passing and other lane changes.”

In the United States, IIHS researchers analysed the driving behaviour of 40 drivers from the Boston, Massachusetts metro area over a four-week period using data collected from the Massachusetts Institute of Technology’s Advanced Vehicle Technology Consortium.

The vehicles provided to the drivers were a 2016 Range Rover Evoque with ACC, and a 2017 Volvo S90 with ACC and Pilot Assist, a partial automation system that combine ACC with lane keeping assist. The systems in both vehicles allowed their drivers to increase or decrease their selected cruising speeds in 5 mph (8 km/h) increments.

Here, data suggested that drivers were 24% more likely to exceed the speed limit on limited-access highways with the systems switched on. When they did exceed speed limits, the margin beyond the posted limit was also greater when the drivers were using the assistance systems compared to when they were driving manually.

When driving both manually and with ACC or pilot assist, sampled drivers in the study were also found to have exceeded to speed limit by the largest margin in zones with a 55 mph (88 km/h) speed limit, in which they were found to exceed the limit by 8 mph (12.8 km/h) on average, compared to exceeding 60 mph (96 km/h) and 65 mph (104 km/h) limits by 5 mph (8 km/h) on average.

Leaving aside any other effect these features may have on the risk of crashing, the study claims that users of adaptive cruise control and partial automation systems are at about 10% greater risk of a fatal crash, according to a common formula for calculating probable crash outcomes. This study did not analyse real-world crashes, the report said.

“Driving faster is more dangerous. You can’t argue with physics,” said Monfort. However, the report also added that the study did not account for other factors that have been shown to reduce crash frequency and severity; for example, it is possible that drivers who selected a higher speed of operation could have also selected a greater following distance, the report suggested, adding that adaptive cruise control systems are designed respond sooner and more smoothly than human drivers when traffic slows ahead.

Future research is required to fully understand the technology’s impact on safety, the IIHS report observed. Making these systems more restrictive in their operation might be the answer, provided that limiting the maximum speed or linking the systems to posted speed limits doesn’t not discourage risky drivers from using adaptive cruise control altogether, it suggested.