There are 3 types of public sewers that may be under our roads:
Foul - foul & wastewater from WC, sinks, washing machines, dishwashers, etc. Discharges to a watercourse
after processing at a treatment plant.
Surface - rainwater runoff from roofs and paved areas. Discharges
directly to a watercourse.
Combined - foul, waste and surface water. Discharges to a watercourse
after processing at a treatment plant. As far as I know about 2/3rds of our sewers are combined.
You would have to ask your local authority to determine what is under the street outside your house. If you snowfoam, wash etc. on the driveway, all that wastewater goes into the surface water system on your property, which then discharges to the public sewage system. If your street has a combined sewer it will be treated. If there are separate foul and surface water sewers, it will NOT be treated and will pollute a watercourse.
To be prudent you should at least empty your washbuckets into the gully where your kitchen or bathroom waste pipes discharge, NOT the gully under the rainwater pipes. You will then be sure that the water will be treated. This is also why councils can fine homeowners (quite heavily) who install dishwashers etc. and connect the waste pipe into a rainwater gully.
I assume in Germany they don't use combined sewers hence the strict rules.
....But how does oil, petrol/diesel, get into the surface water drainage system when you are washing your car? 
It would be oil, coolant etc. that has dripped out of your poorly maintained car and is then carried into the surface water system by the washwater.
Surely there would be more likelihood of oil etc being dropped from long term parked cars (if they leak).
This is then washed into the street gullies and into either a surface water or combined sewer. Street car washing just adds to the volume of contaminants possibly entering our watercourses if there isn't a combined sewer.
