Comment by Moru

The reindeer is not randomly showing up out of the forest. They are herded. The people herding them is putting up plenty of signs. But the reindeers are not on the actual road the whole time so people stop bothering about the signs. And some just drive really fast and realize too late that the warning signs in the cars manual that says "This function does not stop the physical laws from working." might be correct.

The raindeer is not affraid of cars, they just stand there doing nothing until they get hit. It might be a big herd with hundreds of animals on the road and another thousand in the surrounding forest so not much place to run either.