The two noticed that based on the speed limit of the road they were traveling on, the birds took off at differing distances. For example, on a 50-kilometer-per-hour road, birds took off later than on a 110-kph road. This happened independently of a vehicle's actual speed. There was also a differentiation based on whether it was spring or fall. The birds took off later in the spring, but earlier in the fall.
New Scientist reports that the researchers' conclusions are based on cars as a sort of predator. Birds know where the predators are at, and as higher speeds generally make for a more danger, our winged friends learn what places are more dangerous. As for the seasonal change, the scientists argued that birds in the spring are both more active, while younger birds are just starting to leave the nest and are still learning about the dangers of cars. In the fall, this isn't the case.
Head over to New Scientist for a full look. It's an interesting read on how our cars change the world around us in not so visible ways.