Google has unveiled four years' work on their driverless car, but the tech giant was not able to work out a mechanism allowing a driver to take over in an emergency.

According to the New York Times, Google had to nix the steering wheel entirely to go forward with producing about 100 of the self-driving vehicles. To fully take a human driver out of the picture, Google has made these cars without gas or break pedals.

The only human controllable aspect is summoning the car with a smartphone app, an emergency stop button and a start command. The car is designed to pick up its passenger and drive to a location already selected via Google Maps.

The car is a product of the Google X laboratory and it appears to be an even more compact version of the Smart Car.

The decision to all but remove the driver completely was intentional. Sergey Brin, Google co-founder and a lead researcher on the self-driving car, told the NYT it is not reasonable to expect a passenger to take over in an emergency situation. The passenger is most likely not paying attention to the route in the same way that a driver would, so the person would ultimately be lost in an emergency anyway.

Christopher Urmson, a former Carnegie Mellon University roboticist and car project director, said the team aw stuff that made us a little nervous" while testing the vehicle. People could even be asleep while the car is driving them, but there were still no crashes during the tests.

The cars can sense its surrounding about 600 feet in all directions, but still has rearview mirrors to meet California's car codes. These vehicles are not meant to be used on any sort of highway and their top speed has been capped at 25 miles-per-hour. Google said an unspecified Detroit auto-maker will produce the cars, which will be ready by next year.

They will have a range of about 100 miles and their motors will be electric, the NYT reported, similar to the Fiat 500e.

"Obviously it will take time, a long time, but I think it has a lot of potential," Brin said. "Self-driving cars have the potential to drive in trains much closer together and, in theory, in the future at much higher speeds.

"There is nothing to say that once you demonstrate the safety, why can't you go 100 miles per hour?"