About 15 percent of the vehicles are trucks, and therein lies the bridge's special problem. Every time a heavy truck crosses, the supporting structure holding up the bridge shakes, leading to cracks, bent rods, loose bolts and nuts. The vibrations can actually bend metal. It's much the same effect as bending a paper clip several times in opposite directions, say engineers. Eventually the paper clip snaps in two.
Federal funds pay for bridge repairs.
Costs continue to rise, and eventually the entire bridge would have to be rehabilitated, requiring several lanes to be closed for a year or two. Although a new bridge will relieve today's congestion, it will not necessarily speed future trips for commuters, engineers say. Beltway traffic is projected to increase 45 percent in the next 20 years, so the greater capacity of a new bridge will make rush hours then about like they are today.