I appreciate the sentiment, but is the gunk on the car not the gunk that would have washed down the drains in the first place? Clearly there are plenty of nasty cleaning agents, but one supposes that safe ones might be used (I actually spent a while considering exactly this, and it's possible but not super easy getting an environmentally friendly car cleaner). I guess the issue is it's hard to tell from mere drive-by observation what cleaner is being used.

I use Dawn dish soap in hot water. No need for harsh or expensive car wash detergents.