Voice interfaces make it easier for drivers to tune the radio, adjust the air-conditioning, or send a text while speeding down the road. But even though these systems are advertised as allowing drivers to keep their hands on the wheel and eyes on the road, new evidence suggests that they might be more distracting than previously realized.

A study conducted by researchers at MIT with support from Toyota’s Collaborative Safety Research Center found that using voice commands for simple tasks, like finding a radio station or changing the climate settings, is quicker and less taxing for a driver. But using voice controls to perform more complex tasks—such as tuning the radio—often takes longer than doing those things manually. It can also cause drivers to glance away from the road to select from a menu or confirm that the system has recognized their speech correctly.

There’s a “great role for voice” in cars, says Bryan Reimer, a research scientist at MIT’s Age Lab who carried out the research with colleagues. “But now we understand it’s not cost-free.”

The study involved asking research subjects to drive a 2010 Lincoln MKS with voice-activated controls. Reimer stressed that the car’s voice interface is not unsafe and that the results do not reflect a problem with this specific interface; they are more likely representative of issues with voice-command interfaces across the industry. However, the results could help automakers refine their designs as car interfaces become increasingly computerized, connected, and complex.

The study “shows that voice interfaces can be visually distracting, so drivers may underestimate what they can safely do while driving,” says Paul Green, a research professor in the Driver Interface Group at the University of Michigan Transportation Research Institute.

Studying driver attention is already a very important part of road safety research. The National Highway Traffic Safety Administration estimates that more than 90 percent of all road accidents in the United States involve some form of driver error, and that texting and driving played a role in 18 percent of fatal accidents in 2010.

Carmakers have added voice controls to address concerns over distraction, and to address the increasing complexity of more capable in-car entertainment, navigation, and communication systems. “A large part of the industry has focused on voice as a hands-free, eyes-free mode of interaction, and it’s more complicated,” Reimer says.

Another study, published in June by the AAA Foundation for Traffic Safety, concluded that voice interfaces can be mentally distracting even when they don’t require drivers to look away from the road or fiddle with manual controls.

Thomas Dingus, director of the Transportation Institute at Virginia Tech and an expert on vehicle interfaces, says the MIT study backs up previous research on voice interfaces. But he says further research—involving people driving their own cars for extended periods—is needed to determine how drivers adapt to such interfaces over time. “The belief is that there’s not really a safety risk if [the voice interface] is well-designed,” Dingus says. “But we’re trying to figure out what that means.”

The relationship between interfaces and drivers will become more complicated as new autonomous driving capabilities appear in cars. The issue will be how to best turn a driver’s attention back to the road when an autonomous system needs to hand back control (see “Proceed with Caution to the Self-Driving Car”). 

The MIT study is being announced today at the LA Motor Show, where Toyota will demonstrate a research vehicle designed to measure driver distraction. Another study conducted by Toyota and researchers at Stanford, also being released at the show, highlights a new kind of driving simulator, one designed to help explore behavior in autonomous vehicles.