“About 33,000 people die on America’s roads every year. That’s why so much of the enthusiasm for self-driving cars has focused on their potential to reduce accident rates,” says Chris Urmson, technical director of Google’s self-driving car programme.

But a report suggests that autonomous vehicles on the road today are experiencing more accidents than cars with human drivers.

Brandon Schoettle and Michael Sivak at the University of Michigan Transportation Research Institute (UMTRI) compared the US safety record of self-driving cars made by Google, Delphi Automotive and Volkswagen with that of conventional vehicles.


The researchers found a total of 11 accidents involving self-driving cars between 2012 and September 2015, two of which resulted in injuries. With the cars having driven about 1.9 million kilometres on public roads (the overwhelming majority by Google’s fleet), that works out at just over 5.5 accidents per million kilometres.

The good news is that autonomous cars weren’t found to be at fault in any of the crashes. The accidents were also generally less severe and there were no fatalities. However, vehicles driven by humans averaged just 1.2 crashes per million kilometres during the same period, according to police statistics.

Mandatory reporting

Unlike humans, robot motorists don’t have the luxury of letting minor accidents go unreported. The US National Highway Traffic Safety Administration estimates that 60 per cent of fender benders, and even 24 per cent of crashes causing injuries, go unreported. But in California, where almost all autonomous driving in this study occurred, manufacturers of self-driving cars must report every broken taillight and scuffed bumper to the state.

Taking underreporting into account only brings the accident rate for normal vehicles up to around 2.5 crashes per million kilometres – less than half that of self-driving cars.

And being driven by a robot might be even more dangerous than that figure suggests, warn Schoettle and Sivak. “Self-driving vehicles have thus far been driven only in limited (and generally less demanding) conditions, for example, avoiding snowy areas,” they write in their report. “Also, it is unclear to what extent any of these vehicles have performed night-time driving on public roads.” California also has safer roads than most of the US, typically reporting fewer traffic fatalities than the national average.

Schoettle and Sivak didn’t look at why self-driving cars are so accident prone in their report. “One could speculate that the sensors are distracting regular drivers, or that the cars may not be driving as a human would,” says Anuj Pradhan, a behavioural scientist also based at UMTRI.

There is another glimmer of hope for Google, Delphi and Volkswagen. Because their cars have driven so few kilometres and had so few accidents, the researchers concede their margins of error are large enough that the actual accident rate for self-driving vehicles could be similar to, or even lower than, that for conventional vehicles.

Only time, and a few more dented robo-cars, will tell.

Image credit: Kim Kulish/Corbis