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12 FIGURE 7 Transit share of regional transportation emissions (Source : "Greening Mass Transit & Metro Regions: A Synopsis of the Final Report of the Blue Ribbon Commission on Sustainability and the MTA," Metropolitan Transportation Authority, State of New York, Feb. 2009, p. 20). At the regional level, an expansion of well-used transit she drove their own car. Commuter rail systems and subway service will tend to increase GHG emissions from transit as systems free up even more space on the road. The scale of more miles are traveled by transit vehicles, but total trans- the benefit per vehicle depends on the passenger load. The portation GHG emissions will shrink as new transit riders Urban Mobility Report finds that transit reduces congestion- leave their cars at home. Figure 7 illustrates this relation- related delays an average of 31 million hours in each of the ship. When transit service is well used, more transit service country's 14 largest urban areas. In 2005, public transporta- increases transit emissions, but decreases emissions from tion reduced congestion-related combustion of gasoline by the rest of the transportation sector. The net effect is to lower 340 million gallons. A Science Applications International total transportation GHG emissions. Corporation (SAIC) study estimated that saving that amount of gasoline is equivalent to reducing GHG emissions by 3 MMtCO2, or twice the amount emitted annually by trans- CONGESTION MITIGATION portation in Washington, DC (4,8). Roadway congestion is an additional source of GHG emis- sions from transportation. Vehicles burn fuel not just when COMPACT DEVELOPMENT they are traveling, but also when they are idling in traffic. Driving at slower than optimal speeds also burns extra fuel Transit systems are associated with compact development and therefore emits extra GHG emissions. The Texas Trans- patterns. An extensive body of research finds that areas with portation Institute's (TTI's) Urban Mobility Report esti- higher population and employment density typically have bet- mates that congestion consumes an extra 120 million gallons ter public transportation systems than areas with lower popu- of gasoline annually on average in each of the nation's 14 lation and employment density (10). Transit systems tend to largest urban areas (9). That figure translates roughly to 1 be more robust and more highly used in compact urban areas. MMtCO2 per urban area. Transit stops in compact areas provide access to more destina- tions, including workplaces and shops, and are therefore more Transit reduces congestion on roadways by taking private convenient to use than stops in other areas. In compact areas, vehicles off the road. A full bus or light-rail car takes up more people can also live within easy access of transit stops, less space on the road than each passenger would if he or allowing transit systems to attract a higher ridership.