Google today said it would issue regular reports offering some details of crashes involving its driverless cars. Google released its first of what it said would be monthly reports on a website dedicated to the driverless car project. Included in the report is a synopsis written by Google of the twelve accidents since it began testing the robot cars in 2009.

The new reports summarize the project’s activity, data points and any incidents involving the cars. An autonomous vehicle was rear-ended at a stoplight in Mountain View, California, on Thursday, Google said. Last weekend, another unit was hit in the same city. There have been 13 accidents since the project began six years ago, and none have been the fault of the vehicle itself, the company said.

"We’ve made a lot of progress with our self-driving technology over the past six years, and we’re still learning," the company said in its report for May. "Every day we head out onto public streets so we can keep challenging and refining our software."

Google is making its case for driverless cars with more transparency amid questions about the progress of the program, which is part of the company’s X research lab.

Consumer Watchdog has repeatedly called on Google to release the official reports. The public interest group says that the Internet giant must release official accident reports that include not only the Internet giant’s version of what happened, but what the other drivers and any witnesses say.

Crash reports are essential to understanding how the robot cars interact with human drivers, which likely will be the biggest challenge the vehicles will face, Consumer Watchdog said. In most of the crashes the Google robot cars were rear-ended. That could mean that the vehicles tend to stop more quickly than human drivers expect.

Another interesting fact learned from the report was that in two of the crashes the human driver assumed control as it was happening.