SAN FRANCISCO - Google's self-driving prototype cars have taken to streets in the Silicon Valley city of Mountain View, where the company has its headquarters.

The move comes after Google's internal testing of the bubble-shaped vehicle over the past year and more extensive experience with technology adapted for existing cars.

"We want to understand what it really means to have self-driving vehicles in the world - both how people in the community perceive and interact with them, and what the practical realities are for us in operating and maintaining them," Google said in a statement on Thursday.

Prototype cars built from the ground up to get around safely without human drivers join Google's fleet of Lexus vehicles augmented with sensors and other computing gadgetry to manage autonomously on roads.

The prototypes will be limited to 40kmh in Google's hometown and have "safety drivers" who can take over using manual controls if needed.

"We've had 20-plus Lexus vehicles driving on Mountain View city streets for the last few years, but the arrival of our new self-driving vehicle prototypes marks the start of a new phase of our project," Google said.

The Google car uses the same technology as its fleet of Lexus SUVs, which have logged some 1.6 million km.

Google says its self-driving cars were not at fault in any of the dozen or so accidents that they have been involved in.

AGENCE FRANCE-PRESSE