on the flat issue,
if you don't change a flat on the roadside, then your walking right?
i wouldn't want to be sitting there changeing a flat, but i don't think your any bettter off walking.
If you can change a flat standing you might be safer (less profile and more visible.)
On a side note, what about getting the local cycling advocacy org to push for a street sweeper to hit those bridges once in a while.
My experience in bridges is similar, huge ammounts of dirt, glass etc. They're never cleaned.