[alistairSH]

Horse poop degrades fairly rapidly (relative to omnivore and carnivore excrement). Doesn't smell as bad either.

Cars produce all sorts of chemical compounds that ruin the air we breath.

[astrodust]

No. It doesn't degrade rapidly. It stays there for weeks unless there's rain. It does smell bad. Hitting a fresh one is a cyclists worst nightmare.

I'm not saying cars are innocent here, but at least they don't poop.