Google's self-driving cars have been involved in two more crashes, taking the total number of accidents to 13.
Google revealed last month that its driverless vehicles were involved in 11 crashes in the six years that they have been on the road.
Weeks later, this figure has increased to 13 after the company admitted that the vehicles were involved in two further crashes in the space of seven days.
"We just got rear-ended again yesterday while stopped at a stoplight in Mountain View," a Google spokeswoman confirmed to Ars Technica.
Google stressed that neither of the latest incidents, like the other 11, were the fault of the vehicle and were to blame on human error.
"That's two incidents just in the last week where a driver rear-ended us while we were completely stopped at a light," the spokesperson added.
"So that brings the tally to 13 minor fender-benders in more than 1.8 million miles of autonomous and manual driving, and not once was the self-driving car the cause of the accident."
Google revealed more details about the first of these two latest incidents in a report released to show the progress it is making with autonomous vehicles.
"A Google Lexus model AV was travelling southbound on Shoreline Boulevard in Mountain View in autonomous mode and was stopped behind traffic at a red light at the intersection of Shoreline Boulevard and El Camino Real, the report said.
"A vehicle approaching from behind collided with the rear bumper and sensor of the Google AV. The approximate speed of the other vehicle at the time of impact was 1mph. There were no injuries reported at the scene by either party."
Despite this growing number of accidents, Google is confident about the project. "We’ve made a lot of progress with our self-driving technology over the past six years, and we’re still learning," the firm said.
"Every day we head out onto public streets so we can keep challenging and refining our software."
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