Ford autonomous car
Ford's self-driving cars are starting to navigate themselves along snow-covered roads Ford

Ford is now testing its autonomous cars on snow-covered roads, a test seen as one of the toughest for self-driving cars to overcome. The announcement was made at the Detroit Auto Show on 11 January and was joined by a video showing the car in action.

The cars use their lidar systems to scan objects above the road and compare these to maps when the snow-covered road surface cannot be seen. Until now, self-driving cars relied primarily on being able to see the road ahead and its white markings; with these covered, the Ford prototypes scan for landmarks above the road, like buildings, to navigate.

Once landmarks have been found by the lidar sensors and cameras, the car looks up the same objects and buildings on high-resolution maps stored on the car's hard drive. These are then used to drive safely on the correct part on the road and give way at what otherwise would be unsighted junctions. Of course, there are limitations - the system only really works on roads the car has driven on before in better weather and is familiar with.

Ford's global headquarters in Michigan sees long winters with regular snowfall, making it the idea test bed for teaching autonomous cars how to drive in poor weather – something Google, which tests similar vehicles in California and Texas, cannot do. Instead, Google has taken advantage of some rare Californian rain to teach its own fleet of cars to be more cautious in wet conditions.

The company's monthly autonomous car report for December said: "Our cars can determine the severity of the rain, and just like human drivers they drive more cautiously in wet conditions when roads are slippery and visibility is poor. For now, if it's particularly stormy, our cars automatically pull over and wait until conditions improve.

"To explore even more challenging environments, we're beginning to collect data in all sorts of rainy and snowy conditions as we work toward the goal of a self-driving car that will be able to drive come rain, hail, snow or shine." Google is now covering between 10,000 and 15,000 autonomous miles every week with 41 vehicles.

Google and Ford were expected to announce an autonomous car partnership at CES in early January, but this did not happen; instead, it is now thought to be happening at the Detroit Auto Show.

A video published by Ford shows its Mondeo-based prototype driving itself around Mcity in Michigan, a vast outdoor test site with road networks and mock towns set up for autonomous cars to drive around. The news comes just days after Ford said at CES it plans to triple its fleet of self-driving vehicles; the company has said multiple times it expects autonomous cars to be available to consumers by the end of the decade.