Since September, when self-driving cars were allowed to operate on California roads, four of the 48 autonomous vehicles have been in minor accidents. Three of the cars were Lexus SUVs operated by Google, and the fourth was owned by Delphi Automotive.

None of the accidents were serious or caused injury, and two of them were reportedly caused by human error.

The details of each accident aren't known, since California law allows traffic collision reports to remain private. But critics advocate that companies promoting self-driving vehicles should be required to be forthcoming about accidents.