Just read "The Tipping Point" by Malcolm Gladwell. It has been proven that people will behave worse in "trashy" neighborhoods then they would in "nice" neighborhoods (i.e., think of the general population and how they would behave in a church as opposed to a major sporting event).
My suggestion is to plant freeway ivy (or similar plants) to cover the walls and force taggers to go elsewhere as perpetual painting over graffiti is not the answer either. Freeway landscaping is the answer...I hope someone in our city government reads this.
PS - I know I'm gonna take some crap for my "pitbull" comment - but statistics show that while these dogs may NOT bite more than other breeds, they do maim or kill more (when they do bite)...so no, I'm not a proponent of ownership of these dogs. I think pitbull ownership is a status symbol and people need to get over it. I don't think the risk warrants it.