Maybe you grew up with your dad washing the car on the lawn in the summer so the grass would get the run-off water. It was genius! You got the car and clean and watered the lawn at the same time. While not the most environmentally friendly method, it turn out dad was on to something. Washing your car on gravel or the lawn minimizes run off allowing the water to be absorbed into the ground. When you wash your car on pavement all that gas, oil, exhaust fumes, not to mention the chemicals in the wash you are using, flow straight into storm drains that release directly into Puget Sound. Commercial car washes are required to drain their wastewater into sewer systems so it gets treated before being discharged back into the environment. They also use less water than we do at home. This is likely due to the super-powerful, high-intensity nozzles commercial car washes use and probably because we have to pay more if we need more car washing time.
But what if you have to wash your car at home? Again, lawn or gravel is the best surface to use and be sure to use a green car washing soap. A search on Amazon brought up over 200 green car washing products so you are sure to find one you like. Also, if your car is not totally dirty, spot wash or use a water-less cleaner. Using a nozzle on your hose that controls the flow of water will also help and when you are done, dump the dirty water in the sink.
Sources: http://www.care2.com/greenliving/3-big-reasons-why-you-should-not-wash-your-car-at-home.html