Despite continuous efforts to perfect the technology, Google’s self-driving vehicles have hit another large roadblock.

In good weather conditions, when the sun is shining and the road is dry, these driverless cars can transport themselves to mostly anywhere. Unfortunately, in less desirable conditions like rain or snow, the vehicles cannot function properly. Recent tests of the vehicles in poor weather conditions show that they are not capable of telling the difference between types of precipitation. Because of this, the vehicles cannot determine an appropriate speed for the types of weather they encounter.

Another serious issue is the detection systems on the vehicles. Things like rocks, plastic bags, and debris on the road all show up as pixelated images. As a result, the car won’t be able to tell the difference from a small boulder or a plastic bag, and will take extreme measures to avoid both.

It will be a while before driverless cars will be safe and reliable enough to sell to consumers, if that is even possible.