Human drivers were at fault each time there was a crash involving a self-driving vehicle, a report from the University of Michigan’s Transportation Research Institute found. The report released Thursday surveyed autonomous cars produced by Delphi, Google and Volkswagen (including Audi).

Researchers found driverless cars were more likely to be involved in a crash than conventional vehicles, though the human occupants were likely to be injured less severely. The study tracked 11 autonomous vehicle crashes from 2012 to 2015, with 72.7 percent of the crashes occurring when the car was stopped or driving slower than 5 mph. Still, the researchers cautioned there’s much work to be done to get a full comparison.

“First, the distance accumulated by self-driving vehicles is still relatively low [about 1.2 million miles, compared with about 3 trillion miles driven annually in the U.S. by conventional vehicles],” lead author Brandon Schuttle wrote in the study’s abstract. “Second, self-driving vehicles were thus far driven only in limited [and generally less demanding] conditions [for example, avoiding snowy areas]. Therefore, their exposure has not yet been representative of the exposure for conventional vehicles.”

The report comes after a Google autonomous car was involved in a first accident that caused an injury in July. A Google sensors-equipped Lexus SUV was rear-ended in Mountain View, California, and the four people involved in the crash complained of whiplash or neck and back pain.

This also comes amid growing questions about advancing autonomous technology. The biggest: What happens if a driverless car needs to decide who lives and who dies?