Since most vehicles are made from steel and most wildlife ain't, i'd try to detect the small magnetic shifts the iron/steel will produce. You might fail to detect nascars, and get false readings on any deer with piercings ;-), but hey, what do you expect for a few dollars. Philips make a range of small magnetoresistive sensors(KMZxx). I'd use one with built in set/reset coils, KMZ51 springs to mind(or the dual KMZ52). There are some appnotes on Philips website.

/Ingvar