However, they may finally be getting closer.
Google's self-driving cars have now clocked up 300,000 miles without a single accident.
The cars have driven themselves through 'a wide range of traffic conditions' during their travels which - with the Earth's circumference a shade under 25,000 miles at the equator - means they have made the equivalent of 12 round-the-world trips.
'To provide the best experience we can, we’ll need to master snow-covered roadways, interpret temporary construction signals and handle other tricky situations that many drivers encounter.
'As a next step, members of the self-driving car team will soon start using the cars solo (rather than in pairs), for things like commuting to work.
'This is an important milestone, as it brings this technology one step closer to every commuter. One day we hope this capability will enable people to be more productive in their cars. For now, our team members will remain in the driver’s seats and will take back control if needed.'
Urmson added: 'And while these team members are commuting, many of them will be testing our algorithms on a new type of vehicle we’ve added to the self-driving car family over the past few months to help us refine our systems in different environments and on different terrain: the Lexus RX450h.
'With each breakthrough we feel more optimistic about delivering this technology to people and dramatically improving their driving experience. We’ll see you on the road.'
The department said, after conducting demonstrations on the Las Vegas Strip and in Carson City, that the car is as safe - or perhaps safer - than a human.
For one thing, the engineers programed the car to create a ‘virtual buffer zone’ around obstacles, making it more aware than some drivers about their surroundings.
As Urmson said, there's 'a long way ahead', but rumours from within Google suggest the cars could come to the market within three to five years.