Mass transit is a transportation service available to the public for trips generally within metropolitan areas. As with all transportation services, an energy source is a critical input in the production of mass transit trips. Almost all of this energy consumption is derived from burning fossil fuels, a process that emits pollutants affecting human health, visibility, vegetation, and climate change. Transit service is increasingly being scrutinized for its pollutant emissions. Mass transit facilitates travel within densely developed, large urban areas. Because of the existence of mass transit, more intense development within an urban area can occur. Higher-density land use may enable reduced energy consumption when considering the settlement area as a whole.
FORMS OF MASS TRANSIT
The term "mass transit" generally refers to passenger vehicles that are common carriers in urban areas, as distinct from intercity travel. The terms "public transit" or simply "transit" also are frequently used. The major types of public transit are bus (rubber-tired vehicles), rail (running on tracks), and ferryboat. Within each type there are several subcategories.
A rubber-tired, self-propelled transit vehicle using an internal combustion engine for power. Most use direct-ignition (diesel) engines, but gasoline, propane, natural gas, and other fuels are used as well. Buses are available in varying sizes and capacities, varying from 10 passengers to 150 or more passengers, including articulated and biarticulated vehicles. They can be designed for city service (fewer seats, more doors and standing room) or long-distance express service (often with no standees permitted). Buses can operate on most streets open to traffic, including expressways, and also can be operated on exclusive rights-of-way such as high-occupancy-vehicle (HOV) expressway lanes, bus lanes on city streets, or busways with on-line stations.
When powered by an electric motor, generally taking power from overhead wires ("catenaries") such a vehicle is called an electric trolleybus or a trackless trolley. An electric bus taking power from current in the ground has been developed by Ansaldo-Breda. Battery-powered buses also have been developed, although all of these have been smaller than standard-size
|Mode||Vehicle Distance Operated||Passenger Boardings|
|Rail rapid transit||19%||30%|
buses. In the late 1990s, "hybrid" electric-internal-combustion buses were placed in service. These vehicles have a smaller than normal internal-combustion engine, which continually recharges a battery pack. The batteries power an electric motor, which provides supplementary power when needed. Fuel cells, which can be thought of as chemical batteries, also are used on an experimental basis to power transit buses.
Streetcar or Tram
The streetcar or tram, powered by electricity from overhead catenaries and running on rails on city streets, was the backbone of mass transit service from 1890 until the widespread deployment of the diesel bus in the early post-World War II period. In general, the only streetcar lines in North America that survived the transition to the bus were those in high-demand corridors, or those operating in their own right-of-way, such as a roadway median, tunnel, or subway. Since the 1970s, there has been increasing deployment of new streetcar service, now referred to as light rail transit (LRT). These services tend to use larger, articulated vehicles and frequently operate partly on exclusive rights-of-way such as disused rail corridors or expressway medians.
Rail Rapid Transit
Also known as metro, subway, elevated, underground, or heavy rail, this higher-capacity rail service is distinguished by its use of trained vehicles (several
|Public Transit Bus||Passenger Car|
|BTU per vehicle mile||BTU per passenger mile||passenger mile per vehicle mile||BTU per vehicle mile||BTU per passenger mile||passenger mile per vehicle mile|
|% change 1970-97||20%||75%||-31%||-37%||-26%||-16%|
cars attached together) and third-rail electric power (a live connection at grade). Because of its unprotected power source, rail rapid transit cannot be operated on city streets, and runs in elevated or underground rights-of-way. Unlike suburban rail, rail rapid transit service generally operates within cities and on a frequent schedule. Some of the new rail rapid transit systems, such as BART in San Francisco, have a metropolitan scope and distant stop spacing, and thus are similar to suburban rail.
Also known as commuter rail, this type of service generally provides long-distance—50 km (31 mi.) or more—routes to the far reaches of a metropolitan area using railroad rights-of-way. These operations use either diesel or electric power. Service is primarily oriented toward commuters from suburbs to central city locations and is concentrated in the peak commuting hours. Express bus operations also provide similar services, without the need for a separate right-of-way.
In metropolitan areas adjacent to bodies of water, ferries can be used as mass transit—that is, providing frequent service useful for local travel. Geography limits ferryboat use to a select group of cities. Twenty-one cities in the United States and its territories have transit ferryboat service. Another example is Vancouver's SeaBus, which provides very frequent shuttle service between the center city and North Vancouver.
Table 1 shows the frequency distribution of U.S. mass transit service supplied (vehicle distance operated) and service consumed (passenger boardings) by transit mode in 1997. The official U.S. transit statistics also include the "demand response" mode. This type of transit consists of minibuses or vans operating by request, rather than on fixed routes, and typically available only to a portion of the public, such as elderly or disabled people. As shown in Table 1, demand response accounts for 12 percent of service operated but only 1 percent of boardings. Buses accounts for the majority of both service supplied and service consumed. Removing the few largest rail systems from the totals would reveal even greater significance of the bus mode.
IMPACT OF MASS TRANSIT ON ENERGY CONSUMPTION
The potential of mass transit to provide transportation services with low energy consumption relies on the high capacity of transit vehicles, since these vehicles have higher energy consumption per vehicle distance traveled compared to private motorized vehicles or nonmotorized modes. Therefore the occupancy rate of transit service is a key factor in determining its energy efficiency. This rate can be measured by the ratio of person distance traveled to vehicle distance traveled.
The energy efficiency of transit per passenger distance traveled depends both on its usage rate (person travel per vehicle travel) and its fuel efficiency (fuel consumption per vehicle travel). The transit vehicles with the greatest potential for energy efficiency gains
|Year||Transit Bus||Transit Rail||Passenger Car||Intercity Rail||Intercity Bus|
may not be those that can carry the most people per vehicle. Rather, the greatest potential gains can be had by matching vehicle capacity to travel demand. In fact, a high-capacity mode that is little used can increase energy consumption relative to the passenger automobile. On the other hand, a minibus with ten passengers may represent a significant reduction in energy use compared to the case of those ten passengers traveling by car.
The data in Table 2 dramatically illustrate the difference between vehicle fuel intensity and passenger fuel intensity from 1970 to 1977. (Fuel intensity, fuel use per distance traveled, is the inverse of fuel efficiency.) In 1970, the transit bus mode in the United States used 50 percent less energy per passenger mile than the automobile mode (passenger cars, not including light trucks). By 1997, transit buses used 19 percent more energy on average per passenger mile. How did this happen? The fuel efficiency of transit buses declined by 20 percent, while fuel efficiency improved by 37 percent for passenger cars. Occupancy declined in both cases, but twice as much for transit buses. The net result was a significant reduction in fuel use per passenger mile for cars despite declining occupancy, and an increase in fuel intensity for transit buses.
The evolution of transit bus compared to transit rail energy intensity in the United States from 1970 to 1997 is shown in Table 3. Transit bus and rail had similar energy intensities in 1970, but by 1997, energy use per passenger mile had increased by 33 percent for transit rail and by 75 percent for transit buses. This increase is yet more significant given that energy intensity for passenger cars declined 26 percent over the same period and given that intercity rail and intercity bus operations also showed energy intensity reductions of 33 percent and 17 percent, respectively.
In 1997, the intercity bus was the least energy-intense of the modes shown in Table 3. Its energy intensity per passenger mile was one-fifth of transit bus energy intensity. The vehicles used for the two operations are similar, although operating conditions are different. Intercity buses operate mostly on expressways, and transit buses mostly on local streets. Still, the fact that transit bus energy intensity increased 75 percent while intercity bus energy intensity decreased 17 percent suggests that changes in operating patterns, rather than changes in technology, accounted for much of the difference in energy use.
In the United States in 1997, transit accounted for a small fraction of transportation energy consumption, largely because it played such a small role in the total travel market, representing only 1.8 percent of trips. Transit accounted for only 0.7 percent of transportation energy consumption (bus 0.4%, rail 0.2%, and suburban rail 0.1%).
Transit represents an increase in energy use compared to walking or bicycling. For countries with a significant amount of nonmotorized transport, increasing transit use may mean increasing energy use. However, the increased energy consumption may be associated with significant improvements in urban passenger transport, which can produce large economic benefits. It is generally easier for transit modes to compete successfully with nonmotorized modes than with the private automobile for new passengers. But as demand for higher-speed travel increases, those customers may switch to private motorized vehicles.
COMPARISON OF MASS TRANSIT MODES
In the largest metropolitan regions, such as in Tokyo, New York, London, and Moscow, the huge demand for transit makes rail transit both indispensable and energy-efficient. But because transit energy efficiency is so dependent on matching supply with demand, the greater flexibility of the bus mode may have an advantage in this aspect. Bus service characteristics can be easily altered as to time of service, location of service, and size of vehicle used. For rail transit, not much change is possible in the short run.
The rail modes require a significant investment of both fiscal resources and energy in infrastructure,
|Year||Diesel||Gasoline||Compressed Natural Gas||Liquefied Natural Gas||Methanol||Propane (Liquid Petroleum Gas)||Other||Total||Total|
including track, power supply, right-of-way, stations, and structures. This investment can produce a higher quality of service, but it commits the urban area to use of the rail mode and its relatively large vehicles.
The bus mode can use a broader range of vehicle sizes and does not require any separate infrastructure. Where right-of-way is shared with other vehicles, much of the energy used in producing the facilities may have been expended without consideration of use by mass transit. Where there is a need for exclusive bus infrastructure to bypass congested areas, busways can be constructed to improve service.
It is common outside of Europe and North America for bus service to be provided by private operators. The higher service efficiencies often achieved by private transit providers can translate into greater energy efficiency. While contracting out of transit service is possible for rail, it has been more common for bus operations.
Why has the load factor for transit decreased? Transit demand has declined due to higher incomes, higher automobile ownership, and a decrease in the share of jobs and population living in areas that are strong transit markets. At the same time, transit supply has increased, spurred by the growth of government subsidies for transit operation and capital investment. The long-standing problem of intense peaking of transit demand means that large vehicles are needed for only a few hours during the day, only to run nearly empty during off-peak hours. Transit agencies in the United States have been generally reluctant to use smaller vehicles. Privatized transit operations outside of London, England, were quick to adopt minibuses. Transit operation in the United States has largely become regionalized, with providers' operating areas spreading out over vast territories. Because all taxpayers in the region are typically required to contribute to transit via designated sales, property, or motor fuel taxes, transit agencies feel a political obligation to provide at least a minimum amount of service in all parts of the region, no matter how weak the transit demand.
Most urban rail service is electric-powered and most urban bus service is diesel-powered, although diesel rail and electric bus operations do exist, as noted above. The efficiency and environmental impacts of electricity depend greatly on the source of electric power. Although electric vehicles produce no tailpipe emissions, generation of electricity can produce significant emissions that can travel long distances. For example, coal-powered electricity plants produce particulate emissions that travel halfway across North America. Urban buses also can be powered by a variety of alternative fuels.
Applications of engineering and computer technology have the potential for reducing fuel use, greenhouse gas emissions, and toxic emissions from mass transit. They also may help in increasing transit load factors. Technology that enables greater operating efficiency of transit also has the potential to reduce fuel consumption per passenger distance traveled. The introduction of electronic fare cards in New York City prompted the development of new fare policies, including free transfer from bus to rail. The result was a 36 percent increase in bus use between 1997 and 1999. With only a 9 percent increase in bus service during the same period, the fuel efficiency of transit increased dramatically.
Traffic signal preemption and other technologies that seek to speed transit can have a similar effect. Faster bus travel means less fuel consumption per vehicle distance traveled. However, faster transit service increases demand for transit, potentially increasing the transit load factor.
The "conventional" fuels used for transit applications include gasoline, diesel fuel, and electricity. Alternatives to these fuels have been sought to reduce energy consumption, pollutant emissions, greenhouse gas emissions, and use of imported fuels. The conventional fuels for internal-combustion engines are the most energy-dense fuels: petroleum and diesel fuel.
Alternative fuels often are handicapped by the need to develop an alternative fueling infrastructure. Most transit fleets have their own fueling stations, so they are prime candidates for early introduction of new fuels. And transit agencies have experimented with many different fuels. Alcohol-fueled engines proved unreliable for heavy-vehicle applications. Los Angeles found that methanol and ethanol corroded bus engines, scrapped some of the vehicles, and converted the remainder to diesel in 1998-1999. The most popular alternative fuel has been compressed natural gas (CNG). This fuel offers lower pollutant emissions than diesel, but at a higher total cost than diesel. In addition to requiring new vehicles with natural-gaspowered engines and storage areas for fuel cylinders, using CNG requires an investment in new fueling and compression facilities and modifications to depots and maintenance facilities. Considering fueling, inspection, and maintenance, operating costs typically are higher for CNG than for comparable diesel vehicles.
Between 1992 and 1998, alternative fuels increased from less than 1 percent to more than 5 percent of total mass transit fossil fuel consumption in the United States (see Table 4). The share of alternative fuel consumption that was CNG increased from 19 percent to 72 percent over the same period.
One promising new technology is the hybrid electric vehicle, which combines an internal-combustion engine and an electric motor powered by batteries. The engine recharges the batteries, and the electric motor provides additional power during acceleration. These vehicles do not have the severe range and capacity limitations of battery-powered electric vehicles and do not have long refueling (recharging) times. Hybrids equipped with regenerative braking are well suited for city bus routes requiring extensive stop-and-go driving.
Ambient particulate matter (soot) has been linked to increased mortality. Diesel engines are high emitters of particulates. Particulate is also formed in the atmosphere from other pollutants, including those emitted by gasoline engines. As the public has become more aware of this problem, diesel engines have become much cleaner. By adding emissions control devices and electronic controls and by modifying engine designs, a large reduction in the emissions of particulate matter and nitrous oxides (precursors to both soot and smog) has already been achieved. In 2000, the U.S. Environmental Protection Agency proposed new regulations requiring very-low-sulfur diesel fuel and further large reductions in particulate and nitrous oxide emissions. Low-sulfur fuel reduces sulfate particulate emissions and also enables the introduction of emissions control devices that would be damaged by high-sulfur fuel. These regulations require diesel vehicles that have an emissions profile similar to today's CNG buses.
In the longer term, more exotic technologies, such as fuel cells powered by hydrogen, may be feasible. These technologies are far from being economically feasible, but rapid progress is being made. However, as conventional vehicles become cleaner, the relative emissionsreduction benefits from alternative fuels declines.
LAND USE AND SOCIAL AND POLITICAL FACTORS
Matching the supply of transit service to its demand is the primary determinant of the energy efficiency of transit. The demand for transit depends on income levels and land use. As average income rises, so does the value of travel time, and therefore the cost of time spent traveling. Higher incomes also make automobile use more obtainable.
Land use is the other key determinant of transit use. Public transit requires a concentration of trips in the same time and place. A concentration of residential and commercial land use, such as that typically found in cities, is necessary to generate significant transit demand. Concentrated land use has another, perhaps more important effect. To function efficiently, the private automobile must be provided with a significant amount of space for both storage and movement. In many cities there is insufficient space for the automobile, making automobile travel expensive. The largest costs of urban automobile travel are storage (parking) costs and travel time costs, due to traffic congestion. Under these circumstances, mass transit modes can compete effectively. They provide travelers with similar or lower total travel time (including time spent walking to and waiting for transit vehicles), and the avoided cost of parking may be several times greater than the cost of the transit fare.
However, most North American urban development since 1945 has been designed around the space needs of the automobile. A generous quantity of off-street parking is required for all new residential and commercial development. Streets are designed to be wide enough to accommodate free-flowing traffic, even in peak periods. Expressways have been constructed to connect all parts of the metropolitan region. The result has been the development of urban areas that are convenient for motoring. The same set of changes also has made them inhospitable to any mode of daily transport other than driving. The low population density of residential development makes postwar neighborhoods difficult to serve by public transit. More important, the prevalence in North America of widely scattered office parks with ample free parking has ensured that transit cannot compete for trips to those destinations, since driving is inexpensive and transit is either not available or takes up to twice as long.
Cities that were large (1 million or more) before 1920 have cores with a large concentration of jobs, expensive parking, and a large transit share of trips. The New York metropolitan area is the premier example, accounting for nearly 40 percent of all U.S. transit trips. Although newer areas have been able to increase transit ridership, none has transit shares approaching what is still common in the older large cities.
Per capita transportation energy consumption in a city such as New York is much lower than the U.S. average as a result of much lower car use. Although high-quality public transit service is one explanation for this result, parking costs, bridge and tunnel tolls, and the convenience of walking are equally important. Public transit is vital for the transportation needs of New York City residents. But many of the trips that residents of the typical U.S. metropolitan area take by auto are taken by New Yorkers on foot, or not at all. It is the lower number of auto trips, rather than a one-for-one substitution of transit trips for auto trips, that has a large impact on reducing energy use. Transit service in New York City is well used, making it energy-efficient despite the slow speed of surface transit travel.
In the United States, outside of the core areas of older major cities, transit has become the transport of last resort. There is a substantial social stigma attached to using transit, due to the low income levels of transit patrons in most U.S. cities. Transit customers also sometimes fear their vulnerability to crime, especially while waiting at bus stops.
Many transit trips involve transfers between transit vehicles. All else being equal, passengers would prefer not to transfer. However, the mode of access to transit is of interest. The vast majority of transit customers walk to transit and then walk again to their final destination. It is difficult to develop high transit demand in an urban environment that is not conducive to walking.
Park and Ride
A recent trend is the development of park and ride areas for transit passengers to drive to transit. These facilities help make transit accessible to a wide geographic area. Because emissions control devices are ineffective when cold, the auto trip to the transit station may produce nearly as many pollutants as a direct trip to the final destination. Further, park and ride lots occupy a lot of space, making transit stations unfriendly to pedestrians and reducing the opportunities for station-area real estate development. Finally, those driving to transit must still pay the full costs of automobile ownership. In fact, the only reason they are likely to take transit is if the cost of station-area parking and transit fare is less than the parking cost at the final destination. This explains why park and ride areas are most successful on transit lines serving urban cores with high parking costs. Most workers in North America, however, pay nothing out of pocket for parking at work.
Bicycling and Transit
Bicycling to transit is another solution for providing access. With no fossil fuel consumption and no pollution, bicycling to transit is attractive from an environmental point of view. Personal fitness is a major motivation for many bicyclists. Public health authorities concerned about physical inactivity have started to promote bicycling. Bicycle access to train stations is very popular in Japan and the Netherlands and some places in Germany. Since bicyclists can travel faster than local transit on some routes, the bike and ride option is most attractive for accessing express services such as suburban rail or express bus. Bike stations that provide bike storage, rental, and repair services and changing facilities adjacent to transit stations have been developed in three California cities.
Permitting bicyclists to take bicycles aboard transit vehicles allows them to use the bicycle for the trip to their final destination as well. Although many transit agencies are reluctant to make room for bicycles during crowded peak hours, railcars designed with bicycle storage have been deployed in Europe and North America. In the 1990s, hundreds of buses in North America were equipped with bicycle-carrying racks.
Despite the energy, emissions, and public health benefits of combining bicycle use with transit, few people in North America take advantage of this combination. Many urban roads are designed for high-speed operation and have narrow lanes. These designs often scare bicyclists from using the roads, a fear sometimes reinforced by motorists and police who do not believe that bicyclists have a right to use roads. Secure, sheltered bicycle storage at transit stations is rare, and therefore the threat of bicycle vandalism and theft deters others from using bicycling as an access mode to mass transit.
Paul M. Schimek
See also: Batteries; Behavior; Bicycling; Diesel Cycle Engines; Diesel Fuel; Electric Vehicles; Emission Control, Vehicle; Engines; Fuel Cells; Fuel Cell Vehicles; Gasoline and Additives; Gasoline Engines; Hybrid Vehicles; Locomotive Technology; Petroleum Consumption; Railway Passenger Service; Traffic Flow Management; Transportation, Evolution of Energy Use and.
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