Drivers who use cruise control and speed-limiting devices know they can provide real benefits, particularly driver comfort and compliance with speed limits.

But a new study (PDF) suggests a downside to these aids. Drivers have less control overtaking other vehicles and managing the direction of their own vehicles, and have longer reaction times.
The study, which measured the effects of cruise control and speed-limiting devices on driver vigilance and behaviour, was released by the French based VINCI Autoroutes Foundation for Responsible Driving.
“The less work the driver has to do, the less alert he will be behind the wheel,” said Bernadette Moreau, General Delegate of the Foundation, which researches hazardous driving behaviours. “It is widely known that these tools are very effective to maintain safe speeds, but call for user savviness and awareness” to be safe.

Driver attentiveness is a hot-button issue worldwide, with various governments pondering legislation that would supplement bans on texting while driving with strict limits, or outright bans, on using internet-enabled features while their vehicle is in motion.
The study indicated that by automating control of the vehicle, there is a decline in drivers’ attention and control, which reduces their ability to respond to hazards.
For example, when cruise control and speed limiters were used, drivers showed reduced ability to merge into traffic due to greater difficulty in modulating vehicle speed. The aids also caused drivers to remain in the overtaking lane for longer periods of time and to move back into the slow lane less often. Drivers straightened their vehicles less often when using these devices, and had substantially slower reaction times, especially in emergencies.
These behaviours grew more pronounced with the duration of travel, especially when using cruise control. Generally, the reduction of alertness and control was greater when using cruise control than with speed limiters, the researchers said.
The research was conducted by the Centre d'Investigations Neurocognitives et Neurophysiologiques, a laboratory operated under auspices of the University of Strasbourg and the state-owned Centre National de la Recherche Scientifique.

Research was conducted in three separate sessions in 2012. Ninety men and women, equally represented, in ages ranging from 18 to 60 and older, participated using a driving simulator. They were required to alter speed in response to four different scenarios under different driving conditions: using the speed limiters; using non-adaptive cruise control; and controlling the vehicle without the use of driving aids.
The study also found that episodes of drowsiness occurred more frequently when cruise control was utilised (and to a lesser extent, with speed limiters) than when drivers controlled vehicle speed.   
Moreau said that to her knowledge, the study was the first of its kind. An earlier study by the foundation on drowsiness behind the wheel indicated that the average French driver has an hour less sleep each night than 15 years ago, and in the days leading up to vacations, many have even less sleep than usual. Given cruise control and speed limiters are often used for longer trips, the impacts of sleep deprivation found in the earlier study, combined with the findings of the newly released one, suggest caution when using these aids before extensive travel.  
The foundation recommends that driving aids be switched off in certain situations, such as when traffic is dense or when approaching speed-reduction areas like construction zones or toll stations. And during long-distance trips, when the tools are often in extended use, drivers should take rest breaks to recover attentiveness.  
“These are tools drivers can use but they need to use them at the right time,” Moreau said. “These tools are meant to assist, not replace, drivers.”
The full study will be available in English in September.