Google revealed that, for the first time, one of the prototypes for its self-driving car was involved in an injury accident.

The July 1 accident involved a Lexus SUV, which Google outfitted with cameras and sensors, being rear-ended in Mountain View, which is the home city of the tech giant. Over 20 prototype self-driving cars are being tested in Mountain View.

The Lexus self-driving car prototype had three employees of Google as passengers, and they said that they suffered from minor whiplash. The employees underwent a check up at a hospital and were all eventually cleared to return to work.

The driver of the car that rear-ended the Lexus self-driving car prototype also suffered from back and neck pain.

It is a requirement in California to have a person sitting on the driver's seat of a self-driving car that is being tested along public roads, so that the driver can take control of the car in case of an emergency. Google usually also sends a second employee to sit on the passenger seat in front to record the data from the tests, and in the incident, a third employee was sitting at the back seat.

Google, which has been a pioneer in the development of self-driving cars, said that the accident was the 14th one over a span of six years across 1.9 million miles of travel. The company, however, claims that the self-driving cars have not caused any collisions, and in 11 of the 14 incidents, the vehicle was rear-ended.

A blog post by Google self-driving car program head Chris Urmson said their vehicles are surprisingly often being hit by drivers that are distracted, with the clear them being the inattention of drivers and human error.

The accident report which was filed by Google to the California Department of Motor Vehicles stated that the Lexus self-driving car prototype was travelling at 15 mph in self-driving mode behind two cars as it moved towards an intersection. As the two cars in front stopped, so did the self-driving car. However, within around a second, a fourth car that was going at around 17 mph rear-ended the Google vehicle, with the on-board sensors showing that the driver of the offending car did not put on the brakes.

The rear bumper of the Lexus self-driving car prototype was damaged slightly, while the rear-ending car lost the front bumper.