I found this rather interesting and thought it would be worth sharing. The full article is over at The Truth About Cars]Deadly Threat: A Deer In The Headlights | The Truth About Cars
"...Deer, he told me, are prey animals who usually live in the deep woods and they rely a lot on sound to assess threat. A car closing in on them does not make a lot of noise because, thanks to the Doppler effect, it compresses the sound waves in front of it. This is also, as a side note, why someone walking on a train track can be easily run down by a train from behind and why, despite many bikers assertions otherwise loud pipes do not save lives. Out the back and off to the sides however, a vehicle does make noise and these noises are often reflected off the trees as it passes. The deer, whose ears are much more sensitive than the one you and I have, interpret these sounds as coming from the woods and as a result will almost always run up the road and away from a sound."