As a long time wheelchair user who has used BART since its inception, I find the elevators a horrible excuse for a transit system. I hate that visitors to the Bay Area I love experience this. You need to keep in mind that I don't just get it on my shoes, but it gets on my hands. My hands touch my tires that have to run through the awful mess and believe me although the availability of hand sanitizer has helped, it doesn't really do anything for the fact that I can't get what I have on my hands or very expensive gloves out of my thoughts.
Now, I also want to say that a large number of the people who are using the elevators as urinals do not really have an alternative, because in our far-sighted wisdom have chosen to make being without a home (and thus a bathroom) illegal. If we did as many metropolitan cites in other counties and provided public toilets we could solve the problem.