Do you think that they chased the guy, dragged him into the middle of the road and just waited there? What was their reason to wait? Why hold off on killing the guy until they hung him over the bridge?
It was clear from the previous episode that the Saviors have been watching Alexandria. The party Carol ran into were out scouting and knew where she came from.
Clearly they have had pockets of scouts out observing, and used the walkie-talkies to let Negan know when people left and the direction they were headed. That is how they easily caught Daryl, Michonne, Glenn, and Rosita. It is how they herded Rick and the RV where they wanted them so that they could surround them. There is a reason that the last road block had the trees, because Negan's group blocked it way in advance because they knew that at some point Rick would make a trip to Hilltop. That is why the numbers kept getting bigger at each encounter. They knew all the possible routes and blocked the quickest with the smallest group so that if Rick attacked that small group and kept going, the larger numbers were free to hit Alexandria. If Rick didn't force the issue, they knew they could move in to block the other routes and catch them where they had no chance to fight.