I don't even have a driveway, I just do it on the double yellows outside my house. Its a fairly quiet road though. I use the Kleers portable jetwash. http://www.kleers.co.uk/index.php?route=product/product&path=65&product_id=66
Its not that powerful* and the reservoir just about lasts a wash and a rinse
*My shower was out of action this weekend so filled up the kleers with warm water and gaffer tapped the nozzle to the edge of the shower cubicle. Refreshing!