BOX 3.1

Specific Challenges and Opportunities for Transportation Systems

Underground transportation systems will benefit strongly from technical advances as discussed throughout this report. In design and construction, for example, new lining and underground construction technologies are needed that reduce material use and improve long-term facility performance. Underground transportation systems in major cities, however, usually represent key infrastructure elements that are pivotal in terms of the urban mobility that sustains the economy and provides quality of life and hence have a special importance in terms of underground space use. Because they are large public investments and subject to many policy and funding constraints, underground transportation systems may not be designed, operated, and maintained for their maximum contribution to overall urban sustainability. The construction of major underground transportation projects often requires significant relocation of in-situ underground utilities along public rights of way. However, the major excavation work and relocation needs of the project provide key opportunities for renewing and rationalizing utility provision in an area to provide for easier future maintenance of those systems. While this represents an extra burden on the transportation project, it can provide an overall benefit to the urban community using a system-of-systems analysis rather than a project-by-project analysis. Furthermore, in a planning context example, the long-term sustainability of an underground transportation system is improved when system designs allow as much flexibility as possible, taking into account future uses, potential for additional transportation lines, and intermodal connections. This again can increase initial costs but provide for better long-term sustainability.

express arterials and highways, and grade-separated dedicated freight movement corridors for railroads or trucks. High Speed Rail (HSR) service that includes both above- and belowground components is common in Europe and Asia. Each system has unique characteristics to suit its purpose and location. All will likely improve quality of life and long-term sustainability benefits to the urban center(s) served (Jehanno et al., 2011).

Underground transportation, as described in the next sections, can serve to increase community resilience against many natural or manmade hazards including earthquakes and acts of war than their surface counterparts. Box 3.2 provides an example of the performance of transportation infrastructure crossing San Francisco Bay following the Loma Prieta earthquake in 1989. Different types of underground transportation elements and systems and their roles in sustainable urban development are described.

Underground Urban Roads and Highways

Overloaded and congested urban surface arterial roads can be relocated to aerial or underground alignments to obtain grade separation (e.g., transportation routes at multiple elevations) and exclusive rights-of-way. This can relieve the



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