Small device? In a ventilation building? You damage the equipment so that it quits, then stall traffic somehow. Next step, Cyanide or Ricin. Or even a suicide van inside the tunnel. Pop the tunnel's cap, and flood it, panic would be instantaneous and people would be very frightened indeed. Just what the Terr's want. So we can't be frightened and can't change our activities, because they'd win. But if we don't change something, they win anyway because we make it easy for them to do so. This makes me wonder what is being plotted for our mal-enjoyment.
If I remember right, great thought WAS used at the time of the design. The Bay Bridge is like 20 miles long. The bridge portions, while relatively short in themselves, handle incredible shock energy from wind and sea. It is incredibly strong. Even so, the bridge is frequently shut down because that energy is translated into vibration of the bridge structure, which throws large, top heavy vehicles all over.
The Bay Bridge itself services traffic from nearly the entire Eastern seaboard of the United States. New England, NYC, Philly, Wilmington, Ocean City...route 13 is the only route for the residents of many areas to travel back and forth.
When I needed to get to DC I didn't use the bay-bridge...I had a route less traveled and I didn't have to pay the toll. Made better time for less money....but most people ust the bay bridge tunnel for the convenience.