Nets are suspended beneath the bridge to catch chunks of falling concrete. When the net gets too many holes, the city replaces the nets instead of repairing the structure. It doesn't have to be that way. Take the Squirrel Hill Tunnel, which the Highway Users Alliance targets as the source of the region's worst traffic congestion. Otherwise, PennDOT has done nothing to speed traffic since the tunnel opened 46 years ago.
Drivers slow down as they approach the tunnel as a matter of natural behavior.
Inside, there's the claustrophobic effect. Both actions combine to cause the wavelike reaction that produces the miles-long traffic queues during the inbound and outbound rush hours. I suggested in a September 1998 issue of the Post-Gazette's Benchmark series that PennDOT speed traffic a bit by widening the lanes inside the tunnel, thereby alleviating fears and perceptions about being squeezed into lanes that are no more narrow or no less safe than the lanes outside. We're not talking rocket science here. We're talking about relatively cheap, fast modifications that can be made during off-peak hours.