If I had been making the call I would have blown the whole bridge up while the bad guys were parked on it.

The old mayor's argument was they are going to need the bridge in the future so he didn't want to blow the whole thing up. So if that is your reason then I think they should have hidden those explosives just on the other side of the bridge inside some old cars. When the bad guys went to try move it out of the way blow them to bits. In a real TEOTWAWKI those guys would just come back later when your guard is down. Plus by letting them go you just shoved your problem off onto other less armed towns.