to utilities without the need for digging in the street, are designed to minimize subsurface displacements and other Influences that may cause damage to buried and aboveground facilities, and are a more efficient use of underground space than are separately buried utilities. A study by researchers in Spain (Riera and Pascal, 1992) found a distinct economic benefit from locating services in a common tunnel when the value of the underground was included in the calculations during construction of the Barcelona Ring Road. In fact, shared utility tunnels are frequently constructed in Europe where narrow rights-of-way and strong centralized decision making have favored their use.

It has proven difficult to develop utilidors as extensively in the United States. Obstacles include the need to abandon investment in existing service infrastructure, concerns about operational liabilities and risk in a shared or co-located utility environment (e.g., water or gas lines in the same tunnel as electric lines), and administrative concerns related to access to utility lines by others. In addition, initial connection costs may be higher than those for dig and place utilities. Operational issues such as risk and security concerns for utilities, if installed in utilidors, could be circumvented with improved sensor and security systems. The viability, value, and benefits of utilidors may be effectively communicated with (1) development of workable scenarios for secure multi-utility facilities; (2) development of workable scenarios for effective transitioning from current configurations; (3) lifecycle cost-benefit analyses comparing separate and combined utility corridors; and (4) demonstration projects. In the United States, utilidors have been built typically as part of major old and new developments or underground transportation improvements (e.g., Disney World in Orlando, Florida, with its extensive underground service “city” and the Chicago freight tunnel network). If the United States is to improve the sustainability of its urban utility services and preserve underground space for more cost-effective sustainability opportunities for future services, then this impasse needs renewed attention.

Underground Transportation Facilities

The long-term sustainability of urban areas is positively affected by the availability of underground transportation systems. Cities such as Singapore have benefited from master plans designed around transportation systems (Hulme and Zhao, 1999). Well-planned underground transportation systems tend to reduce urban sprawl, saving landscapes and protecting biodiversity, and can positively impact land use and development decisions (Bobylev, 2009; Sterling et al., 2012). They provide safe and efficient transportation and decrease the need for and use of automobiles, reducing congestion and travel times, which in turn reduces fossil fuel use and emissions (Besner, 2002).

Underground transportation assets can address multiple growth-related challenges in urban areas, but many challenges also remain to be addressed (see Box 3.1). Today, many cities have urban transit subway systems, underground

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