Not sure where/if it is a written rule/law. But evidently (as it was explained to me) you’re supposed to wash the car in your yard so the run off goes through the ground instead of washing directly down the street drain because those empty straight into the water retention ponds. In fact our pond is right behind our house. I would think though that with environmentally safe soap it shouldn’t matter and not harm the wildlife, but just trying to behave myself.