It has emerged that driverless cars may not work in heavy rain and they could be blinded by sunlight.
It comes as Google is confident the self-driving cars will be on forecourts within five years - and is reportedly in talks with Volkswagen, Toyota and other car makers.
The cars have a sensor on their roof that uses laser-based radar, and it is understood that rain may cause interference.
Advertisement
They also have cameras to read traffic lights, but they can become confused when hit with direct sunlight.
The company wants to begin testing the cars on roads in the US state of California within months, before a larger public project.
Google is aiming to launch the cars in 2020, with something like this:
