One of Google's self-driving car prototypes has been involved in an injury accident for the first time.

A Lexus SUV with sensors and cameras installed by the tech giant was rear-ended in Google's home city of Mountain View, California, where more than 20 prototypes have been self-manoeuvring through traffic.

The three people on board, who complained of minor whiplash injuries, were checked at a hospital and cleared to go back to work following the July 1 collision, Google said. The driver of the other car also complained of neck and back pain.

In California, a person must be behind the wheel of a self-driving car being tested on public roads to take control in an emergency. Google typically sends another employee in the front passenger seat to record details of the ride on a laptop. In this case, there was also a back seat passenger.

According to an accident report Google filed with the California Department of Motor Vehicles, Google's SUV was travelling at about 15mph in self-driving mode behind two other cars as the group approached a junction with a green light.

The first car slowed to a stop so as not to block the junction as traffic on the far side was not moving. The Google car and the other car in front of it also stopped.

Within about a second, a fourth vehicle hit the rear of the Google car at about 17mph. On-board sensors showed the other car did not brake.

The driver of that car reported "minor neck and back pain". The SUV's rear bumper was slightly damaged, while the vehicle that struck it lost its front bumper.

Mountain View police responded, but did not file an accident report.

Google has been a pioneer of self-driving technology, which it believes will be safer and more efficient than human-driven cars. This is the 14th accident in six years and about 1.9 million miles of testing, according to the company.

Google has said that its car has not caused any of the collisions, though in 2011 an employee who took a car to run an errand rear-ended another vehicle while the Google car was out of self-driving mode.

In a blog posted yesterday, the head of Google's self-driving car programme, Chris Urmson, said his SUVs "are being hit surprisingly often by other drivers who are distracted and not paying attention to the road".

In an interview, Mr Urmson said his team was exploring whether its cars could do something to alert distracted drivers before a collision. Honking the horn would be one possibility, but Mr Urmson said he worried that could annoy residents of Mountain View.