Google said it would give monthly updates of accidents involving its driverless cars.
The report for May showed Google cars had been involved in 12 accidents since the company first began testing its self-driving cars in 2009, mostly involving rear-ending.
Google said one of its vehicles was rear-ended at a stoplight in California on Thursday, bringing the total count to 13 accidents.
"That could mean that the vehicles tend to stop more quickly than human drivers expect," public interest group Consumer Watchdog said.
A Google spokeswoman said the consumer watchdog conclusion was erroneous because most of the rear-end accidents occurred when the vehicle was stopped.
Consumer Watchdog called for more details on the accidents, including statements from witnesses and other drivers.