In the City of Portland, for example, streets and highways have a major effect on stormwater runoff

And lets remember just who is paying to take care of that. As has been posted here (thank you Sam and crew) it is paid not by auto users but by sewer rate payers.

The bottom line is that what really needs to be looked at is how much the government has encouraged private auto use, including things like the first Persian Gulf "war for oil", the Interstate system and low density development (is free parking really the best use of land and money?).