Birds are not ‘stupid’.
A blackbird, for instance flys low over a forest clearing because by keeping a ‘low profile’ he/she is less likely to be picked off by a predatory bird such as a sparrowhawk.
The blackbird is doing the same thing when he/she flys low over a road. Unfortunately, it’s going to take some time before evolution kicks in, and the birds figure out that by flying low over a road they are in more danger from a passing car then from a predatory bird.