Though neither company would say exactly how many cars are on the city's streets, they have become a ubiquitous presence in San Francisco as the state gradually lifts restrictions. Cruise currently has a permit to charge for driverless passenger pickups and drop-offs in limited areas of the city from 10 p.m. to 6 a.m. The company also can offer free passenger service throughout the city at any time of day, with or without a safety driver. Waymo can charge for its rides throughout San Francisco with a safety driver present, and can offer fully autonomous pickup and drop-off if the ride is free.

With its Shinkansen'bullet trains' and melodious subway system, Tokyo already has some of the world's greatest public transport infrastructure. But the heavily populated city will be pushed to its limits come 2020, when the world descends on the Japanese capital as it plays host to the Olympic games. One company looking to capitalise on the influx of tourists is robotics firm ZMP Inc. According to Reuters, it's planing to team up with Tokyo's Hinomaru Kotso cab firm to update its fleet of 600 cars with driverless technology. ZMP has already had driverless cars on Tokyo's streets, but each had a driver ready to wrestle control should the AI go wayward. It will begin testing truly driverless cars this year, ahead of its ambitious 2020 goal.