This will be trivial to prove in court because the robot car will be storing video from all cameras, as well as its debug log, in some sort of a "black box." As you, the owner, are not required to pay attention to driving, it will be one machine (the car) vs. another (the government.) If the car could not see the signs, it will be clearly a fault of the government because the robot is an objective judge of what is visible and what isn't, down to the last pixel.
I would rather prefer this scenario because it would be unfair when an LEO accuses the driver of not following the rules when those rules are poorly presented or missing entirely. As a human, you cannot present evidence of what you saw. As a robot, the car can do that. After a short while the police will switch from hunting the drivers to "programming" the cars by maintaining the signage. A passenger in a robot car, just like in a human-operated bus, won't be responsible for following the rules of the road. This will remove a considerable percentage of abuse of power by LEO, as they won't have as much power anymore over the occupants of the vehicle.
My question was not whether the robot car could win in court. My question was, how will they know to pull over? I guarantee you that at some point in some radar trap town, there will be a cop who wants to pull them over. And if sirens can make them pull over on a desolate roadway somewhere, that would be a real cool way to hijack an expensive shipment of electronics.