[squeaky-clean]

My car doesn't die if I don't attend to it for 2 weeks. If my car breaks its "leg" I can swap it out, I don't have to shoot the poor thing. My car can't get me in trouble for grazing in my neighbor's pasture. If horses traveled at car speeds, I doubt death counts would be any lower, and you'd have to figure in the number of horse deaths too.

[willcipriano]

If horses traveled at car speeds they would refuse to do so when it is unsafe. The same way you won't knowingly run full speed down a ice covered street.