[Timesink]

Depending on which bridges you blew up, you could cause major transportation disruptions throughout that local area. If, for example, the Golden Gate became impassable, how would people get across the bay? They'd have to take some pretty lengthy detours, and those detours would just be doubling or tripling traffic on other already-overloaded lanes.

[FITZ]

They've had threats on the bridges into Mexico too, I think it would all be about disrupting transport of goods and travel.

[fleance]

They would probably attack during rush hour. Most of these bridges are bumper-to-bumper during morning rush hour. I hope the coast guard is protecting against boats trying to blowup the bridge supports in the water. The golden gate bridge is held up by two main towers with large underwater piers.

[APBaer]

"If, for example, the Golden Gate became impassable, how would people get across the bay?"

They wouldn't, they would be...
"Sitting on the dock of the bay
Watching the tide roll away
I'm just sitting on the dock of the bay
Wasting time"