This isn't unusual behavior for birds, Ed Yong says over at National Geographic. In fact, a researcher, Pierre Legagneux, devoted an entire study to it, and found that the speed limit of the road actually changes how far and fast birds fly. Yong explains:
One year and 134 measurements later, Legagneux clearly showed that birds flee from incoming traffic at greater distances on roads with higher speed limits. On a 110 km/h road, they'd be airborne when the Peugeot was 75 metres away. On a 20 km/h road, they'd wait till the car was less than 10 metres away before taking off. That's fairly predictable. But more surprisingly, Legagneux also found that the birds didn't react to the actual speed of his car. Their flight initiation distance depended on the speed limit of the road, but not on the speed of the incoming traffic.