Firstly, no one is going to bring a wet sloppy hose in and out of a house in the frozen Northeast winters unless they're a fanatic, and your driveway would become an ice-skating rink afterward. So that leaves a carwash. There is a carwash eight miles from me. Using only the little wand for about five expensive minutes, there's no way I'm going to be able to property and practically reach enough of the undercarriage unless I crawl underneath the truck on the soaking wet ice-cold cement floor, and that's not going to happen. Even if I could clean off the entire undercarriage, with all the salt embedded slop nearly always on the roads here in the winter, it'll just get covered again with salt by the time I get home, so it's entirely moot. It's simply not practical.
And it's simply not a good idea.
Salt reacts with water to cause rust. When you soak the undercarriage and then ride on salted roads, you're creating the ideal environment for rust. There's no way to avoid salt in the wintertime in the salt belt. It's better just to leave it inert as dry or frozen whenever possible than to purposely add water, which is the perfect catalyst to create rust. This is also why cars rust more in a heated garage than staying frozen as everyone knows. Unfrozen moisture and salt
together cause rust. Salt alone won't. You could leave salt all winter on the metal and, unless there is moisture, it won't cause rust. So, it's just not a good idea to add more moisture. When winter is over, and they finally stop spreading hundreds of tons of salt on the roads, then you thoroughly and properly flush out the undercarriage using a pressure washer and a special
undercarriage sprayer for up to an hour to get rid of it all for spring, summer, and fall.
Undercoating is the best and most practical rust preventative for cars in the salt belt. I speak with lifelong experience living here my entire life.