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specific transit stations. One agency, Sacramento Regional · Compressed natural gas (CNG), liquefied natural gas
Transit District, is developing TOD guidelines. (LNG), and propane/liquefied petroleum gas (LPG) --
Specially designed vehicles can burn these types of fossil
TABLE 6 fuels. CNG is the most commonly used in transit buses.
AGENCIES PROMOTING COMPACT DEVELOPMENT (% of 41 · Biodiesel--Biodiesel fuel, made from soy, cooking
respondents) grease, or other sources, can be blended with conven-
Planning or tional diesel and used in standard diesel buses. Some
Strategy Types Planning Implementing Implementing changes to maintenance procedures may be necessary.
Station area · Hydrogen --Hydrogen is an emerging transportation
54% 39% 59%
planning (TOD) fuel. A few transit agencies have hydrogen buses, typi-
Coordination with cally for demonstration purposes.
local/regional · Hybrid propulsion systems --Hybrid systems gener-
61% 44% 71%
development ally supplement a diesel-fired engine with a battery and
decisions electric motor that recapture some energy from normal
Other strategies 0% 2% 2%
vehicle motion and braking. Electric motors can also
be combined with bus engines that burn other types
70% of fuels.
Any strategies
(28 agencies) · Electricity --Electricity is typically drawn from over-
head catenaries by trolley buses, but is also used in bat-
All agencies taking steps to promote compact develop- tery powered electric vehicles
ment patterns or TOD complementary to transit services are
aware that these strategies can reduce GHG emissions. Half TCRP is currently updating its Guidebook for Evaluat-
of these agencies indicated that reducing GHG emissions ing, Selecting, and Implementing Fuel Choices for Transit
was a factor in their decision to promote compact develop- Bus Operations. The revised guidebook will include basic
ment. Sarasota County Area Transit characterized its efforts information on the life-cycle GHG impacts of various alter-
to promote compact development as part of the county's native fuels and on the cost of various fuels.
efforts to promote sustainability. Five agencies--Montgom-
ery County DOT, Sound Transit, LACMTA, Massachusetts Electricity differs from other fuel types in that emis-
Bay Transportation Authority, and San Francisco's BART-- sions do not come from the transit vehicles themselves, but
noted that reducing GHG emissions was a principal factor in rather from the point at which the electricity is generated.
promoting compact development patterns. Emissions depend on the source of electricity. Traditional
fossil-fired generators release CO2 emissions as they burn
coal, oil, or natural gas. Other types of electricity genera-
VEHICLE EMISSION REDUCTION STRATEGIES tion, including nuclear, hydroelectric, wind, and solar, pro-
duce little or no GHG emissions in operation. The GHG
Transit agencies have substantial opportunities to reduce emissions associated with electricity therefore depend on
GHG emissions from transit vehicles by making changes to the specific mix of generation facilities. Transit agencies
transit vehicles, fuels, and operations. Alternative vehicle in regions of the country with relatively low-emitting elec-
technologies and fuels have received particular interest in tricity supplies, such as King County Metro in Washing-
the transit industry, but conventional vehicles and fuels also ton State, benefit from lower electricity emissions. Some
can reduce vehicle emissions. agencies make direct purchases of cleaner electricity from
known generation sources, rather than using the standard
Alternative Vehicle and Fuel Technologies mix from the local electricity grid. Although electricity is
generally considered an alternative energy for transit, it is
For road-based transit systems, alternative fuel and vehicle a standard power source for many rail-based transit sys-
technologies can significantly reduce the amount of GHG tems, including light rail, subways, and some commuter
emissions per mile of vehicle travel. Nearly 80% of U.S. rail systems.
transit buses are powered by conventional diesel engines
(see Figure 11). Conventional diesel-fired internal combus- The use of alternative fuels in transit vehicles has risen
tion engines are one of the most carbon-intensive technolo- sharply in recent years, as shown in Figure 12. Use of elec-
gies that buses can use. An average 40-ft diesel bus with a tricity increased by 18% from 1995 to 2006. Use of CNG
fuel economy of 3.5 mpg emits 6.5 lb of CO2 per mile trav- increased by a factor of 14 over the same period. Consump-
eled (3). Alternative propulsion technologies currently avail- tion of other alternative fuels also increased, in all cases more
able for transit buses include the following: rapidly than consumption of diesel fuel, which grew by 8%.
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FIGURE 11 Distribution of active transit buses by fuel/propulsion system (Source : Neff, 2008 Public Transportation
Fact Book, Part 2 : Historical Tables, APTA, Washington, D.C., June 2008).
FIGURE 12 Alternative fuel consumption by transit vehicles, 19942006--millions of gallons (diesel equivalent)
(Source : 2008 National Transit Database, Fuel consumption table, Federal Transit Administration).
The impact of alternative fuel and vehicle technologies on degree to which alternative bus propulsion technologies and
GHG emissions varies by fuel and vehicle type as well as by fuels can reduce GHG emissions on a per mile basis. These
operating conditions. A number of studies have assessed the studies typically analyze emissions across the full life cycle
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of the fuels, beginning with the production of fuel feedstock. experimented with lighter weight buses. By one estimate, the
Production of feedstock includes growing soybeans, in the use of lightweight materials can reduce fuel consumption by
case of most biodiesel, or extracting fossil fuels, in the case one-tenth of a gallon per mile. The report found that the cur-
of diesel and CNG. Life-cycle assessment also accounts for rently available technology in a hybrid-electric propulsion
emissions from the refining of fuels, transportation of fuels system burning diesel or biodiesel, installed in a lightweight
to the point of distribution, and combustion of fuel in vehi- composite fiber body, is a particularly promising option for
cles. This type of assessment is also known as a "well to low-GHG buses (45).
wheels" assessment.
There is some uncertainty about the extent to which CNG
A 2007 report commissioned by the California Energy buses reduce GHG emissions. The methane burned in these
Commission (CEC) contains the most comprehensive assess- vehicles is also a GHG and, when released uncombusted, has
ment to date of GHG impacts of alternative fuels in buses. a greater GWP than CO2. An empirical trial by the Northeast
That study compared a total of 13 vehicle and fuel combina- Advanced Vehicle Consortium on year 2000 buses found that
tions for buses in California. In addition, it assessed a num- CNG buses produced higher GHG emissions on a simulated
ber of fuel production pathways for each fuel type. The fuel New York City duty cycle, as well as on a central business
pathway, or the process by which a fuel is produced, affects district cycle, than did diesel buses. Vehicle cycles in these
its life-cycle GHG emissions. For example, it takes less areas include slower average travel speeds and more stop-
energy to produce biodiesel from canola than from soy. The ping and starting than cycles in other areas. Some models
origin and destination of fuels is also important. The farther suggest that existing CNG buses produce little to no GHG
feedstocks are transported, the higher are life-cycle emis- benefit over conventional diesel buses; however, improve-
sions. The CEC report uses a number of assumptions spe- ments to CNG bus technologies are expected to offer more
cific to fuel consumption in California. The report assesses substantial benefits in the future (45).
the impact of fuels in various future years, given expected
improvements in vehicle technologies over time (42). In rail transit, regenerative braking is the technology
with the greatest potential to reduce energy consumption
Table 7 summarizes the study's findings on the reduc- and thereby reduce GHG emissions. Regenerative braking
tion in GHG emissions in urban buses using various alterna- systems on rail cars allow vehicles to capture energy as they
tive fuels. Electric vehicles provide the greatest reduction slow or stop and store it for later use or transfer it to vehicles
from conventional diesel, at 55% less GHG emissions per elsewhere in the system. Current technologies only allow
mile. (This figure assumes the average electricity generation the transfer of energy to nearby trains, but with technology
mix in California.) A blend of 20% soy-based biodiesel with improvements, trains should be better able to store energy
conventional diesel reduces GHG emissions the least of the on board (45). Both BART and NYMTA are exploring
options examined, at 12%. Note that results for individual new regenerative braking technologies for their rail transit
transit agencies can vary based on a wide range of assump- systems.
tions. For example, the electricity generation mix in a region
has a substantial effect on the level of emissions associated Almost all survey respondents are either currently using
with electric vehicles. or planning to use alternative vehicles or fuels in their transit
fleets. More than three-quarters are operating or planning to
More recently, several studies have questioned the ability purchase hybrid electric vehicles. About one-third are oper-
of a large-scale shift to biofuels to provide a net reduction in ating or planning to purchase more fuel-efficient vehicles
GHG emissions. Fuels produced from crops, including corn- powered by conventional technologies, such as lightweight
based ethanol and soy-based biodiesel, cause some additional diesel buses. Another third are operating or planning to pur-
GHG emissions from the conversion of natural lands to agri- chase electric vehicles. More than two-thirds of agencies are
cultural lands. Fuels produced from waste products have an using or planning to use alternative fuels in transit vehicles.
advantage in this regard. Taking conversion of natural lands Biodiesel was the most common alternative fuel type cited
into account, some studies have found that crop-based bio- by transit agencies, followed by CNG, electricity, and hydro-
fuels are responsible for more GHG emissions than conven- gen. None of the agencies surveyed are pursuing or using
tional fuels (43,44). The data in Table 6 do not take these LNG or LPG.
additional emissions from land use change into account.
Agencies cited a variety of initiatives to use alternative
TCRP Report 93 assessed the state of research and devel- vehicle technologies and fuels:
opment of various alternative bus propulsion technologies
as well as likely future trends in adoption. The report also · SFMTA and TriMet currently fuel their entire bus
examined the possibility of using lighter materials in buses fleets with biodiesel blends.
to reduce the weight of the vehicle and improve fuel effi- · SFMTA has a goal to convert its entire fleet to electric
ciency. Both Houston Metro and LACMTA have successfully drive vehicles by 2020.
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TABLE 7
REDUCTION IN LIFE-CYCLE GHG EMISSIONS AND PETROLEUM USE IN URBAN BUSES, COMPARED WITH DIESEL FUEL (Year
2012 Vehicles)
Liquefied Compressed
Biodiesel Natural Gas Hybrid Electric Natural Gas
Fuel/Vehicle Type (B20) (LNG) Methanol Vehicle (CNG) Fuel Cell Electricity
Petroleum Reduction from Diesel 16% 100% 97% 20% 100% 100% 100%
GHG Reduction from Diesel 12% 16% 18% 20% 23% 24% 55%
Source: TIAX LLC, Fuel Cycle Assessment: Well-to-Wheels Energy Inputs, Emissions, and Water Impacts, California Energy Commis-
sion, 2007 (42) [Online]. Available: www.energy.ca.gov/2007publications/CEC-600-2007-004/CEC-600-2007-004-REV.PDF.
· AC Transit (AlamedaContra Costa Transit District) TriMet). King County Metro notes that it receives credit for
uses gasoline hybrid buses. use of biodiesel on the Chicago Climate Exchange.
· VTA is currently testing biodiesel in buses.
· RTD plans to use hybrid CNGelectric buses. Operations and Maintenance
· Southwest Ohio Regional Transit Authority is consider-
ing biodiesel derived from palm oil, which may reduce Transit agencies can improve the fuel efficiency of their
GHG emissions more than typical soy-based biodiesel. existing transit vehicles, and thereby reduce GHG emis-
· Foothill Transit, a small agency in the Greater Los sions, largely by improving the operations and maintenance
Angeles area, plans to convert its entire bus fleet to of vehicles. Operational strategies include the following:
CNG and to test electric buses.
· Driver education--Vehicle operators can be trained in
One challenge for some agencies in using alternative fuel-efficient driving techniques, such as smoother accel-
fuels is finding a sufficient supply of the fuel and finding eration and deceleration and avoiding vehicle idling. The
funds to purchase alternative fuels, which sometimes can Canadian Urban Transit Association's SmartDRIVER
be more costly than conventional fuels. The affordability of program has provided instruction on fuel-efficient driv-
alternative fuels can change from month to month with fluc- ing to more than 100 transit system representatives (46).
tuations in petroleum markets and markets for other fuels · Anti-idling policies or technologies --Unnecessary
and feedstocks. Both King County Metro and TriMet report idling of transit vehicles may occur at stations, stops,
that their use of biodiesel has been constrained by cost fac- and maintenance yards. Technologies that automatically
tors. King County Metro is investigating long-term contracts shut off vehicle engines after several minutes of idling,
with biodiesel providers to stabilize the volume and price of or policies that instruct drivers not to idle unnecessar-
their fuel supply. ily, can reduce fuel consumption. New Jersey Transit
is reducing idling of diesel train engines by switching
All agencies pursuing alternative vehicle or fuel technolo- trains to electric power when in railyards.
gies are aware of the impact that these strategies can have on · Maintenance programs--Routine vehicle maintenance
transit vehicle emissions. Of those agencies operating alter- programs can improve vehicle efficiency. Keeping
native vehicle types, more than three-quarters cite reducing bus tires properly inflated is one simple maintenance
GHG emissions as a reason that vehicles were purchased, measure to improve fuel efficiency. In 2005, TriMet
and more than one-third cite reducing GHG emissions as a maintenance crews boosted gas mileage on buses by
principal factor (Montgomery County DOT, Southwest Ohio approximately 10% by adjusting transmissions, front-
Regional Transit Authority, Jacksonville Transportation end alignments, and steering control arms, and main-
Authority, Community Transit, TransLink, Sound Transit, taining a set tire pressure.
LACMTA, Transit Authority of River City, Lee County Tran- · Vehicle retrofits --In some cases, retrofits to existing
sit, Hampton Roads Transit, Sarasota County Area Transit, vehicles may improve energy efficiency and reduce
Massachusetts Bay Area Transit Authority, and BART). GHG emissions. For example, Palm Tran in Palm
Beach County, Florida, is installing electric fan kits
Of the agencies using or planning to use alternative fuels, on bus vehicle engines to improve fuel efficiency.
again more than three-quarters cite reducing GHG emissions LACMTA is considering installing improved batteries
as a reason, and more than one-third cite reducing GHG emis- on their CNG buses to reduce idling, and is converting
sions as a principal reason (Montgomery County DOT, South- some of its buses to run on electric power.
west Ohio Regional Transit Authority, LYNX, Jacksonville
Transportation Authority, King County Metro, Council on Other improvements to bus fleets and operations can
Aging of St. Lucie, TransLink, Lee County Transit, Hampton improve fuel efficiency or reduce the amount of vehicle travel
Roads Transit, Sarasota County Area Transit, Palm Tran, and needed. GPS technologies on transit vehicles can help transit
