I would imagine that a troll wouldn't be interested in a bridge if it didn't have much in the way of foot traffic. If all a bridge carried was vehicles or rail cars, not much for them to eat. Also if the bridge was mostly made out of iron or steel, then it wouldn't be much good for a troll either.
Now if there are areas of reinforced concrete or wood, then they should work for a troll just fine. Likewise if the bridge frequently has poor visibility due to fog, that should give trolls the opportunity to get 'takeout' when they're hungry.