While no injuries were reported at the scene of the accident, Google notes in its monthly report on self-driving car accidents that the driver voluntarily checked himself into the hospital and was evaluated by medical staff and released.
The accident was reported by some news media at the time, but the new Google report provides a more detailed account of the incident, which occurred on September 23 in Google's hometown of Mountain View, California.
According to the report, one of Google's Lexus-model self-driving cars was going through a green light at an intersection on Phyllis Ave. in Mountain View when the car's autonomous technology sensed another vehicle coming towards it that seemed as if it was going to run through its red light. The Google car began applying the brakes on its own, then the driver switched the car into manual mode and took over - but maybe not quickly enough.
What happened next sounds like it was probably very frightening for its human driver: The other vehicle came into the intersection at 30 miles per hour, running a red light and hitting the Google car's right side, effectively t-boning the car. Google said its car was traveling at 22 miles per hour at the time of the collision. Both the Google car and the other vehicle "sustained substantial damage," the report says.
A witness at the time told 9to5Google that the Google employee looked "dazed" while waiting for a tow truck to arrive.
Google did not immediately respond to an inquiry by Business Insider on Thursday about whether the driver was determined to have been injured in any way by the medical staff at the hospital.
The accident is the most serious to date involving a Google self-driving car prototype, which Google has been testing for several years. While the accident appears to have been caused by the other driver, the incident is sure to raise further questions about how safely self-driving cars can be integrated into real-world traffic situations.
Given that the Google car sensed it was at risk of getting hit by another car, it's unclear why the Google car was not able to avoid the situation on its own, rather than requiring the human driver to take over.
Google has said it believes its self-driving car technology will help reduce the thousands of driving accidents and deaths that occur every year.