Hopefully someone will move this to the Detailing section. In short, it's better to wash the car in a jetwash place, making sure nothing but water touches it. No scrubbing brushes etc.
If you create a hazard, you could well be held responsible for any mishaps that occur thereafter eg little old lady slips on the frozen water and cracks her head open.
Alternatively, if the car is well protected with sealant and waxel, youcould use something like Optimum No Rinse, which uses a couple of buckets of water and a sponge (shock horror!) but is safe to use on paintwork without risk of marring.