I got this a couple of years ago for the first time ever and it did freak me out too! I also found the information regarding golfer's vasculitis and other possible causes. By the time we got home after driving, it was almost totally gone.
I did a bit of research on it, and some of the things I've done since then to prevent it are:
a) Taking Ester-C - there is a study out there showing that vitamin C and bioflavinoids (Ester C has both) can help prevent vasculitis. The study used about the equivalent of 2 per day and I just take 1 per day, but I've had luck with it.
b) Avoiding NSAID's - I try to use only tylenol on my trips - NSAIDs like ibuprofen can cause rashes anyway in some people and I've never had an issue with it normally, but on the trip when I got the rash, I was using ibuprofen more than tylenol.
c) Wear high socks - I look like a dork and I don't care. I normally wear no-show socks with sneakers, but now on vacation, I wear Diabetes and Circulatory socks by Dr. Scholl's that come about 1/4 to 1/2 way up my calf. Covers up that part of my leg where I got the rash before, in case a reaction to the sun off the pavement is to blame. (I don't have diabetes OR circulatory problems, but the socks are designed not to be binding for people who do have those issues).
d) Putting my feet up any time I sit down - just simple elevation of the legs
Now, I have no idea if any one of these things, or a combination of all 4, have helped to prevent a re-occurrence, but I have not had the rash again in 2 trips (1 in June, 1 in August) since I have done these things.