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[1] A fuel-based approach is used to estimate nitrogen oxides (NO x = NO + NO 2 ) emissions from gasoline-and diesel-powered motor vehicles. Estimates are made at the national level for the period 1990-2010. Vehicle emissions are also estimated at the state level for California, and for the South Coast (Los Angeles) and San Joaquin Valley air basins. Fuel-based emission estimates are compared with predictions from widely used emission inventory models. Changes in diesel NO x emissions vary over time: increasing between 1990 and 1997, stable between 1997 and 2007, and decreasing since 2007. In contrast, gasoline engine-related NO x emissions have decreased steadily, by $65% overall between 1990 and 2010, except in the San Joaquin Valley, where reductions were not as large due to faster population growth. In the San Joaquin Valley, diesel engines were the dominant on-road NO x source in all years considered (reaching $70% in 2010). In the urbanized South Coast air basin, gasoline engine emissions dominated in the past and have been comparable to on-road diesel sources since 2007 (down from $75% in 1990). Other major anthropogenic sources of NO x are added to compare emission trends with trends in surface pollutant observations and satellite-derived data. When all major anthropogenic NO x sources are included, the overall emission trend is downward in all cases (À45% to À60%). Future reductions in motor vehicle NO x will depend on the effectiveness of new exhaust after-treatment controls on heavy-duty trucks, as well as further improvements to durability of emission control systems on light-duty vehicles.

Public bikesharing-the shared use of a bicycle fleet-has recently emerged in major North American cities. Bikesharing has been found to decrease driving and increase bicycling. But shifts in public transit have been mixed. The authors evaluate survey data from two U.S. cities to explore who is shifting toward and away from public transit as a result of bikesharing. The authors explore this question by mapping geocoded home and work locations of respondents within Washington DC and Minneapolis. Respondents were mapped by their modal shift toward or away from bus and rail transit. The results show that in Washington DC, those shifting toward bus and rail transit live on the urban periphery, whereas those living in the urban core tend to use public transit less. In Minneapolis, the shift toward rail extends to the urban core, while the modal shift for bus transit is more dispersed. The authors analyze sociodemographics associated with modal shift through cross-tabulations and four ordinal regression models. Common attributes associated with shifting toward public transit include increased age, being male, living in lower density areas, and longer commute distances. The authors conclude with a discussion of the final results in the context of bikesharing's impacts on other cities throughout North America.

Carsharing has grown considerably in North America during the past decade and has flourished in metropolitan regions across the United States and Canada. The new transportation landscape offers urban residents an alternative to automobility, one without car ownership. As car-sharing has expanded, there has been a growing demand to understand its environmental effects. This paper presents the results of a North American carsharing member survey (N = 6,281). A before-and-after analytical design is established with a focus on carsharing's effects on household vehicle holdings and the aggregate vehicle population. The results show that carsharing members reduce their vehicle holdings to a degree that is statistically significant. The average number of vehicles per household of the sample drops from 0.47 to 0.24. Most of this shift constitutes one-car households becoming carless. The average fuel economy of carsharing vehicles used most often by respondents is 10 mi/gal more efficient than the average vehicle shed by respondents. The median age of vehicles shed by carsharing households is 11 years, but the distribution covers a considerable range. An aggregate analysis suggests that carsharing has taken between 90,000 and 130,000 vehicles off the road. This equates to 9 to 13 vehicles (including shed autos and postponed auto purchases) taken off the road for each carsharing vehicle.

A fuel-based inventory for vehicle emissions is presented for carbon dioxide (CO 2 ) and mapped at various spatial resolutions (10 km, 4 km, 1 km, and 500 m) using fuel sales and traffic count data. The mapping is done separately for gasoline-powered vehicles and heavy-duty diesel trucks. Emission estimates from this study are compared with the Emissions Database for Global Atmospheric Research (EDGAR) and VULCAN. All three inventories agree at the national level within 5%. EDGAR uses road density as a surrogate to apportion vehicle emissions, which leads to 20-80% overestimates of on-road CO 2 emissions in the largest U.S. cities. High-resolution emission maps are presented for Los Angeles, New York City, San Francisco-San Jose, Houston, and Dallas-Fort Worth. Sharp emission gradients that exist near major highways are not apparent when emissions are mapped at 10 km resolution. High CO 2 emission fluxes over highways become apparent at grid resolutions of 1 km and finer. Temporal variations in vehicle emissions are characterized using extensive day-and time-specific traffic count data and are described over diurnal, day of week, and seasonal time scales. Clear differences are observed when comparing light-and heavy-duty vehicle traffic patterns and comparing urban and rural areas. Decadal emission trends were analyzed from 2000 to 2007 when traffic volumes were increasing and a more recent period (2007)(2008)(2009)(2010) when traffic volumes declined due to recession. We found large nonuniform changes in on-road CO 2 emissions over a period of~5 years, highlighting the importance of timely updates to motor vehicle emission inventories.

Public bikesharing-the shared use of a bicycle fleet by the public-is an innovative mobility strategy that has recently emerged in major North American cities. Bikesharing systems typically position bicycles throughout an urban environment, among a network of docking stations, for immediate access. This paper discusses the modal shift that results from individuals participating in four public bikesharing systems in North America. The authors conducted an online survey (n =10,661 total sample), between November 2011 and January 2012, with members of four major bikesharing organizations (located in Montreal, Toronto, the Twin Cities, and Washington D.C.) and collected information regarding travel-behavior changes, focusing on modal shift, as well as public bikesharing perceptions. The survey probed member perceptions about bikesharing and found that a majority in the surveyed cities felt that bikesharing was an enhancement to public transportation and improved transit connectivity. With respect to modal shift, the results suggest that bikesharing generally draws from all travel modes. Three of the four largest cities in the study exhibited declines in bus and rail usage as a result of bikesharing. For example, 50% of respondents in Montreal reported reducing rail use, while 44% and 48% reported similar shifts in Toronto and Washington D.C., respectively. However, within those same cities, 27% to 40% of respondents reported using public transit in conjunction with bikesharing to make trips previously completed by automobile. In the Twin Cities, the dynamic was different, as 15% of respondents reported increasing rail usage versus only 3% who noted a decrease in rail use. In all cities, bikesharing resulted in a considerable decline in personal driving and taxi use, suggesting that public bikesharing is reducing urban transportation emissions, while at the same time freeing capacity of bus and rail networks within large cities.

By July 2011, North American carsharing had grown to an industry of nearly 640,000 members since its inception on the continent more than 15 years ago. Carsharing engenders changes in member travel patterns both towards and away from public transit and non-motorized modes. This study, which builds on the work of two previous studies, evaluates this shift in travel based on a 6281 respondent survey completed in late-2008 by members of major North American carsharing organizations. Across the entire sample, the results showed an overall decline in public transit use that was statistically significant, as 589 carsharing members reduced rail use and 828 reduced bus use, while 494 increased rail use and 732 increased bus use. Thus for every five members that use rail less, four members use rail more, and for every 10 members that ride a bus less, almost nine members ride the bus more. The people increasing and decreasing their transit use are fundamentally different in terms of how carsharing impacts their travel environment. This reduction, however, is also not uniform across all organizations; it is primarily driven by a minority (three of eleven) of participating organizations. At the same time, members exhibited a statistically significant increase in travel by walking, bicycling, and carpooling. Across the sample, 756 members increased walking versus a 568 decrease, 628 increased bicycling versus a 235 decrease, and 289 increased carpooling versus a decrease of 99 study participants. The authors found that 970 members reduced their auto commuting to work, while 234 increased it. Interestingly, when these shifts are combined across modes, more people increased their overall public transit and non-motorized modal use OPEN ACCESSEnergies 2011, 4 2095 after joining carsharing than decreased it. Data collected on the commute distance of respondents found that carsharing members tend to have shorter commutes than most people living in the same zip code. The analysis also evaluates the distribution of residential population density of members and its association with average changes in driving. The analysis finds that average driving reductions are consistent across population densities up to 10,000 persons/square kilometer but become more varied at higher densities.

Over the past 20 years, China has experienced a steady decline in bicycle use. To address this trend, China's central and local government for urban transportation created Public Transit Priority to encourage public transport initiatives. As part of this effort, the government of the city of Hangzhou launched Hangzhou Public Bicycle in 2008. This service allows members to access a shared fleet of bicycles. As of March 2011, Hangzhou Public Bicycle operated 60,600 bicycles with 2,416 fixed stations in eight core districts. To understand factors leading to bikesharing adoption and barriers to adoption, the authors conducted an intercept survey in Hangzhou between January and March 2010. Two separate questionnaires were issued to bikesharing members and nonmembers to identify key differences and similarities between these groups. In total, 806 surveys were completed by 666 members and 140 nonmembers. The authors found that bikesharing was capturing modal share from bus transit, walking, autos, and taxis. Approximately 30% of members had incorporated bikesharing into their most common commute. Members indicated that they most frequently used a bikesharing station closest to either home (40%) or work (40%). These modal shifts suggested that bikesharing acted as both a competitor and a complement to existing public transit. Members exhibited a higher rate of auto ownership than nonmembers. This finding suggested that bikesharing was attractive to car owners. Recommendations for improving bikesharing in Hangzhou included adding stations and real-time bike and parking availability technologies, improving bike maintenance and locking mechanisms, and extending operational hours.