Now that spring is here this becomes an option. Is it more cost effective to wash a car at home (increasing water utility expense) or to pay to have it washed at a car wash? Also which is more environmentally friendly?
In most of North America, water rates are cheap enough that it will save in actual dollars to do this at home; but there are other considerations.
You might also factor in your own enjoyment; if you like washing the car, more power to you, if you do not, then your time might be better spent doing something else.
The environmental factors are harder. The car-wash probably uses more electricity, and the amount of water is difficult to judge, as it depends on how you wash the car yourself.
One major difference, from an environmental perspective would be that all your soap and other grime and oils, in the city, are all washing down a storm drain and going untreated into your local waterway. The car-wash likely is connected to a sanitary sewer, so the water would be treated before being released.
I tend to take the most cost and environmentally effective approach; and wait for rain. :-)
I think that it is cheaper to wash the car at home and maybe to ask your family members, so you can talk why you are doing it. Also with practice you can do this with the minimal amount of water. On the other hand, if I take my car to a car-wash I tend to go and have a coffee or something while I am waiting, so this again adds to the cost of not doing myself.
I have never used it, but there are products called "waterless car wash" which seems to be some chemicals and wax.
This would use less water and electricity still, but I couldn't tell you how long your $20USD will wash a car. It says 10 car washes, but I would bet your mileage may vary.