This story has been updated to reflect the following: Google says its test cars have driven the equivalent of nearly 100 years of human driving.

A handful of autonomous cars have been in accidents during test runs in California, posing a key question for the future of driverless vehicles: Are they actually any safer?

Four of the about 50 self-driving cars in California have been in accidents since September, the Associated Press reported Monday. Three of the four cars were Google’s Lexus SUVs. The fourth was a Delphi Automotive car.

Driverless cars have been hailed as a technology with the potential to improve safety by removing human error from roads.

Two accidents took place while the cars were driving, but for the other two, humans behind the wheel had control, according to the AP. Under California law, collision reports are private, so the state’s Department of Motor Vehicles didn’t give the AP more information. Google GOOG, -2.37% told the AP the accidents produced minor fender-benders, light damage and no injuries, and were caused by human error and inattention.

“The person familiar with the accident reports said the cars were in self-driving mode in two of the four accidents, all of which involved speeds of less than 10 mph,” the AP says.

In the event that the crashes were due to human error, car makers are faced with the question of how much control to offer drivers-turned-passengers in autonomous vehicles. Self-driving cars are expected to reduce peoples’ insurance costs, given that humans would likely be absolved of most responsibility for accidents. Those costs could shift to auto manufacturers.

Insurers and car makers will also be forced to make moral decisions. What should a car do when a pedestrian walks onto the street and a crash is inevitable: save the walker or save the driver?

Google says its test cars have driven the equivalent of nearly 100 years of human driving, and some accidents can be expected.