Toyota's not alone in the slow lane to self-driving cars

General Motors Co's self-driving unit, Cruise, said earlier this year it was delaying the commercial deployment of cars past its target of 2019 as more testing of the vehicles was required.

In GM's case, unexpected technical challenges, including the difficulty for cars to identify whether objects are in motion, are requiring the company to take more time and caution to develop its technology, sources have told Reuters.

The Japanese components maker Denso, Toyota's biggest supplier, believes it will take years for the technology for fully self-driving cars to hit the roads. For now it is working towards developing cars that can accelerate and brake on their own with driver monitoring.

"The most difficult aspect of developing these driving systems is to anticipate unexpected movements in the vehicle's surroundings," said Hajime Kumabe, the head of Denso's engineering R&D centre in Tokyo.