"Whose idea was it to blow up the bridges?"
"Blowing the bridges was the only thing to do. It was the only way to keep out the infection."
"Yeah they can't get in, but we can't get out."
two survivors arguing in the City Hall
Prior to the events of the Dead Island campaign, Ryder White had planted explosives on the bridge, destroying it. Beforehand, a Raskol gang had stationed themselves on the bridge, hoping to break through the barricade before the demolition of the bridge.
During the Dead Island game, the Heroes are told by Mother Helen that the bridge to wealthy area was detonated before their arrival, in order to, presumably, prevent the poor people from looting the wealthy area.(It is revealed in Ryder White campaign that the actual purpose of detonating the bridge is to contain the infection) Being obliged by this, the heroes then travel through the Sewers to the Town Hall in hopes of getting supplies. On their arrival, Two richmen, including Sony Haoda, are arguing about the demolition of the bridge, stating that doing so has cut off their paths of either running away or getting possible help.
But it is revealed in Ryder White's Campaign, that if the bridge is not destroyed, then the Raskol gang will have broken through the police barriers on the bridge and make it towards the wealthy area, on the other hand opening the path for zombies to invade the other part of the city.