Cars aren't the main problem really. An overpopulation of deer puts a strain on the local ecosystem that nature cannot cope with. A given area of vegetation can support a given amount of herbivores, whose population is naturally checked to near the given amount by predators. Since humans 'meddled' by killing of top predators, there are now more deer across most of the U.S. than the vegetation can support. Unchecked, the deer would strip the vegetation, causing long term ecosystem damage. Also, less food means weaker deer, who would be less likely to survive the winter. Due to the 'meddling' humans took the place of the top predator, and should we stop checking the herbivore population, ecosystems would be changed drastically as vegetation and deer populations plummet.