Drivers are 24% more likely to speed when using adaptive cruise control

Another day, another Insurance Institute for Highway Safety study looking at how people are using advanced driver assistance systems. Like the study we covered on Wednesday, this one also concerns adaptive cruise control (ACC), which uses forward-looking radar to control the distance to a vehicle in front of it (unlike old-fashioned cruise control, which will happily rear-end someone if you don't turn it off). Sadly, the findings are not encouraging. In its study, the IIHS found that drivers were much more likely to speed while using ACC.

Back in the mid-'90s, the National Highway Traffic Safety Administration evaluated the then-new technology (PDF), logging 35,000 test miles (56,000 km). The NHTSA found that "ACC is remarkably attractive to most drivers. The research indicates that, because ACC is so pleasing, people tend to utilize it over a broad range of conditions and to adopt tactics that prolong the time span of each continuous engagement." However, the agency's results also suggested "that ACC usage has induced some elevation in the speeds that would otherwise prevail in conventional (i.e., manual and [conventional cruise control]) driving."

The IIHS notes that ACC is marketed as a convenience feature, not a safety feature, but also notes that some studies have linked it to a decrease in crashes and insurance claims. However, this link may be muddied by the fact that the system is usually used in combination with forward collision warning or automatic emergency braking. For example, the claims that Tesla Autopilot reduced crashes by 40 percent actually turned into a finding that it increased crash rates by 59 percent once properly analyzed.

To test how ACC affected speeding, the IIHS recruited 40 drivers in the Boston area, then gave half of them a 2017 Volvo S90 to use for about four weeks and the other half a 2016 Range Rover Evoque, both of which were equipped with ACC. (The Volvo also featured lane-keeping assist as part of Volvo's Pilot Assist II system, which you can read more about in our review.) The vehicles were equipped with a monitoring system that included a video camera facing the main instrument display (to determine when ACC was operating), as well as vehicle speed and location, and the speed limits. IIHS also only recorded data on controlled-access highways.

Analyzing the data showed that drivers in both the Volvo and Range Rover were significantly more likely (95 percent) to exceed the posted speed limit when using ACC than not (77 percent), although there were no significant differences between the two groups.

However, the absolute differences in speed were not that great. When driving manually, drivers averaged 6.1 mph (9.8 km/h) over the speed limit. When using ACC, this increased to 7 mph (11.2 km/h), or 7.1 mph (11.4 km/h) when using Pilot Assist in the Volvo. Interestingly, drivers sped more on highways with 55 mph and 60 mph speed limits than on 65 mph-limited roads. The IIHS estimates that "[c]ompared with manual driving, the increase in speed associated with ACC/Pilot Assist use was estimated to increase crash risk by 10 percent for fatal crashes, by 4 percent for injury crashes, and by 3 percent for property-damage-only crashes."

The IIHS does note that it did not take into account the following distances while using ACC, which drivers can control (in increments of 1 and 5 mph). It also noted that drivers generally drive 5-10 mph (8-16 km/h) faster than the posted limit, and that's easier to accomplish when using ACC than when driving manually.