Mother nature likes fast drivers
My justification for no speed limit is if a car can go faster, then the amount of time it will be on the road is less, so less time for the car to pollute. For example, if you have a 100 mile trip, and you drive 70 miles per hour,it would take approximatly 1.45 hours. But since almost any new car can top 100 mph, it would take only 1 hour, thats 0.45 hours less burning gas. Even less gas will burn because at higher speeds, engines are more efficient anyway, so quite a bit of gas can be saved, in return less polution overall.
Moral of the story is that if you love the enviornment, drive fast, safely.
