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Chris, you're saying that all street drains never go into the sewage system?

Here in NZ most street drains go straight to the nearest stream. It's illegal to wash your car in your drive in many areas, because the chemicals in the washing and the stuff you wash off go into waterways (washing on your lawn is OK though). It's also illegal to have stormwater runoff pipes (eg from your roof guttering) going into sewers.

The reasoning being basically that storm water should be fairly clean, and doesn't need any further processing. Sewer water however has to be treated, and it's an expensive process. You don't want any unnecessary water going into the system because every drop has to be paid for.

Thanks to some weird and wonderful drain work on a friend's place before he got the property, we've been learning lots about what is and isn't legal with this stuff. Council looks for illegal connections by pumping smoke into the sewer system, and looking for what leaks above ground in the street. If all is connected properly the smoke won't be seen, but an illegal tap (or broken pipe) will let the smoke out.