Typically, bridges and tunnels have a design life three to five times longer than roads and railways and are considerably harder to move once they are in place, making choice of location crucial. Whatever choice of location is made, we and our descendants will have to live with this decision for a long time, perhaps in some cases for centuries. Bridges and tunnels affect land use and land ownership patterns like no other structure. For example, since the 1970s in the Washington, D.C., area around metro stops, clustering of people and Business increased as new stops were added.

The cost to construct and maintain bridges and tunnels is 10-1,000 times greater per mile than that of road and rail, and the construction of these critical links takes a very long time when compared to surface road and rail. In some instances traffic must be completely rerouted away from the construction site, rather than diverted to detours on nearby streets.

Once they are built, bridges and tunnels have the potential for dramatically high levels of traffic, resulting in the potential for significant congestion and unsafe conditions. Important supporting roadway approaches, nearby streets, and ramps must be carefully engineered and monitored to support the increased traffic as drivers converge on the bridge or tunnel from streets and highways. Streets, ramps, and highways exiting the bridge may have their own congestion issues, which can back up onto the bridge. These structures have even been singled out as special targets of terrorists because of their "bottleneck" nature.

Transportation agencies and companies have applied GIS for years for planning, design, construction, operations, and maintenance of roadways and railways, including bridges and tunnels. The two articles in this section point out that bridge and tunnel managers are starting to apply GIS in a special, more intensive way in order to ensure the safe and smooth flow of traffic over and through them. Increased investment in GIS and other technologies is necessary and beneficial to successfully maintain the intensive management of bridges and tunnels.

For more information, vist www.esri.com/transportation on the Web.