Regardless of how people should do something, you're forced to consider what you know, based on experience, they will do. You're not supposed to go off the road and crash your car, but if you do, hopefully you won't run into concrete dividers because a lot of times those have those plastic drum things filled with water or sand or whatever.

Similarly, when one lane at an intersection gets a red light, the other lane's light doesn't turn green for a second or two, just because people go through yellow lights when they shouldn't and so cars could be in the intersection when the light turns red.