I had the exact same problem as Industrialite does. A railroad crossing acts to cut all traffic, and nothing gets over it. Traffic data shows zero traffic on bridges going over railroads, etc.
The thing is, I had the European version of the game, installed in English, which means all my traffic was left-handed, a fact I had never even consciously noticed before.
Aaaand, naturally, I did not install the left-handed bit of the NAM. Having now re-installed with the left-handed stuff included, everything works wonderfully.
That's my mystery solved.