Brb, putting a bridge in my game.
But seriously, this is a very detailed post on such a topic.
Ultimately the bridge in it's simplest form is an object that limits the player's movement. So what I think the best way to do it is to limit even further.
"The heroes step upon on the bridge and are ambushed by bandits that were waiting in the middle of it. Trying to distance themselves from the bandits the heroes try to step back, but oh no the bandits were hiding in the bushes near the bridge, they are approaching from the back."

Suddenly the players are surrounded and their movement is very limited. Perfect place for an encounter.