Any ideas on how to accomplish this?

[Melanie]

There's also Pressure Sensitive Cable... but Luciano's Gay Deer might exert the same point-weight as a vehicle. Then there's a variation such as Piezo Cable... that would give you a different signal profile from a point-weight such as a hoof than from a tyre.

However for a real cheap option, this is my favourite...

[Ron Marcus]

I did some work for a client interested in perimeter detection/identification. He had a magnetic vibration sensor, which was simply a magnet suspended on a spring, inside a coil of wire. It was amazingly sensitive, and could pick up a person walking 50 feet away. There is all kinds of algorythm software to identify vibrational footprints and distinguish between vehicles, wildlife, Al Queda, etc.

Ron

PS...Those deer are probably from Vermont! Civil Union...It's not just a law, it's a Man-date!