This summer some of the self-driving cars will be tested on the roads around Mountain View, California, where the search giant is based.

Before now, the small vehicles have only driven on test tracks and have not mixed with regular traffic.

Google has tested autonomous vehicles on public roads but all of them have been heavily modified Lexus SUVs.

The robot cars will not be completely autonomous, but will have safety drivers on board who can take over if needed.

"Every moment has been building towards putting these cars on the roads where we can start learning even more from them," said Jaime Waydo, systems engineer in the self-driving project, in a video released to accompany the announcement.

eventually the purpose-built robot vehicles will not have a steering wheel or any other control though detachable versions will be used during the forthcoming road tests. The top speed of the cars will be capped at 25mph (40km/h) during the tests.

In preparation for their public debut, said Ms Waydo, the cars had been put through a series of demanding reliability and durability tests. Each vehicle has been clocking up thousands of miles each week on the test tracks - some of which resemble Californian highways and streets.

Mixing with real-life traffic will help Google engineers refine the on-board software to cope with many situations the cars have not encountered during testing, said project head Chris Urmson in a blog post.