Alternative Fuels & Electric
The majority of school buses in the U.S. and Canada run on diesel fuel, but those engines are far different today than the ones decades ago that spewed forth dark, black clouds of soot from the exhaust. And they appear to be changing further, as the U.S. EPA is targeting further reductions of oxides of nitrogen (NOx) that will increase the cost of these engines and vehicles.
Estimates are that about 80 percent of the U.S. national fleet of approximately 480,000 school buses still run on diesel, with the remainder consisting of gasoline and alternative fuels/energy. Informal surveys conducted by School Transportation News indicate that the most widely-used alternative fuel is propane, but electric is closing in. It’s important to note that biodiesel, which commonly ranges in regular diesel-blend amounts from 2 percent to 20 percent, while most often produced from soybeans is not a standalone fuel because it complements diesel. Therefore, it is also not classified as an alternative fuel. Renewable diesel, however, is an alternative drop-in fuel because it is a sustainable, non-fossil fuel created from biomass and can be used in a diesel engine without modifications. It also does not void the warranty.
Propane has gained ground over the past several years, especially in Texas, where the Texas Railroad Commission introduced big incentives. Originally, propane was looked down upon by school bus operators because the system was installed as an aftermarket kit.
That changed when Blue Bird introduced its Type C Vision in 2008. Blue Bird unveiled its second-generation Vision Propane-Powered School Bus during the 2011 STN EXPO in Reno, Nev. In a Jan. 19, 2011 webinar, partner ROUSH CleanTech, which provides the propane auto-gas system that works with a Ford engine, shared on the technical aspects of the Vision as well as a history on propane.
Thomas Built Buses also announced in November of 2010 that it would also make available a propane model powered by Powertrain Integration PythonLI. Propane engine manufacturer Power Solutions International purchased Powertrain Integration in 2015, shortly after becoming the propane partner to IC Bus, a Navistar company.
The U.S. EPA approved the ROUSH CleanTech propane autogas system used by Blue Bird as the first to achieve the optional standard of 0.02 grams per brake-horsepower and hour (g/bhp-hr) of NOx. Emissions are further reduced with renewable propane, or biopropane. ROUSH said renewable propane at the point of ignition is carbon-neutral. Renewable propane made from raw used cooking oil has an average carbon intensity score of 30. By comparison, conventional propane has a carbon intensity of 83, diesel 95 and gasoline 96. The challenge to date, as with renewable diesel, is availability.
CNG appears to be the next-most commonly used alternative fuel in use for school buses, especially in places like California where the Air Resources Board had required for the past two decades that all new school bus purchases be of the CNG variety. That changed over the last several years with mandates that school buses and other commercial trucks be zero emissions.
CNG has traditionally been more prevalent on the West Coast, but in recent years the fuel has expanded to the Midwest. Two of the three manufacturers of large school buses (Blue Bird Corporation and Thomas Built Buses) offer CNG options, and a third manufacturer (Starcraft) has said it would offer a CNG Type A school bus in 2011. A partnership between Cummins, Inc. and Westport also offers an EPA-certified 0.02 g/bhp-hr NOX natural gas engine for school buses. But CNG production has dipped, and electric bus production now outnumbers it.
On the small Type A school bus front, Blue Bird developed a similar propane system for its Type A Micro Bird that runs on a ROUSH CleanTech system developed for the Ford E-450 chassis. Competitor Collins Bus Corporation was the first Type A school bus manufacturer to introduce propane with its solution for the Chevrolet Express and GMC Savana 3500 and 4500 van cutaway chassis, in partnership with Clean Fuels USA. Since, GM announced that it would offer propane power to all Type A school bus manufacturers that build on their built on the Chevrolet Express and GMC Savana 4500 series chassis with Vortec 6.0L engines. Plans to extend the offer to 3500 series chassis was in the works. However, propane supplier CLEANFuel USA was acquired by competitor PSI, which decided to no longer provide propane conversion kits for 2020-year small bus production.
Collins Bus also announced in late 2011 that it would offer the first CNG option for Type As alongside clean energy partner BAF, but interest in CNG has waned since.
Hybrid and plug-in electric vehicles also made their way onto the scene several years ago but have since all but vanished due to price, infrastructure, battery and supply-chain challenges. In 2014, there were less than 1,000 hybrid or plug-in electric vehicle (PHEV) school buses in use nationwide, with the largest fleet in Wisconsin as two school bus contractors combined to run 25 of these vehicles. IC Bus introduced the first PHEV school bus in 2006.
The subsidiary of Navistar offered the PHEV using an Enova Systems drive solution and a charge-sustaining hybrid model utilizing an Eaton system. Thomas Built Buses, meanwhile, offered a charge-sustaining hybrid-electric option in partnership with Eaton, and Collins Bus offers the hybrid option for its Type A NexBus on a Ford E-450 chassis with technology from Azure Dynamics Corporation. Since then, both OEMs have walked away from their hybrid projects.
However, nearly all school bus manufacturers have embraced zero-emissions electric drivetrains, including the Lion Electric Company with its next-generation conventional LionC and transit-style LionD. The Quebec-based company was the first to mass-produce and sell large electric school buses in the U.S. and Canada. In 2018, it also unveiled its LionA electric, flat-nosed mini school bus. Type-A school bus body manufacturer Trans Tech Bus offers its SSTe in partnership with Motiv Power Systems.
Additionally, ADOMANI initially worked with Blue Bus Corporation and in partnership with Electric Drivetrain Integrators (EDI) to unveil a Type D All American Electric prototype at the 2017 STN EXPO. The first Blue Bird All American Electric school buses were shipped to Los Angeles-area school districts in September 2018. Blue Bird and ADOMANI also partnered on a Type C Vision Electric, with both buses expected to be production-ready by the end of 2018. Cummins acquired EDI in 2018 and became the new partner with Blue Bird. School districts began using the Blue Bird electric buses in route operations in early 2019. Meanwhile, the Micro Bird Type A brand also offers an all-electric model, the G5 Electric, with a Toronto-area school bus contractor receiving the first model.
Motiv Power Systems manufacturers the electric chassis for Type A builders Collins Bus and Trans Tech. Collins Bus also partners with Lightning eMotors for an electric school bus.
Thomas Built Buses and IC Bus also unveiled their own all-electric prototypes in late 2017, with production versions released in early 2021. Thomas partners with Proterra on the Saf-T-Liner C2 Jouley electric bus and is moving to a partnership with Meritor, which Cummins acquired, for its BlueHorizon electric drivetrain. But Thomas will continue to use Proterra battery packs.
GreenPower also offers an all-electric Type D school bus as of 2018 and in 2022 unveiled its Type A electric school bus. Prior, the company and A-Z-Bus Sales, Inc. partnered on a project to retrofit old diesel school buses to run on electric, and the Torrance Unified School District in Southern California has been working with funds from the Clinton Global Initiative since 2014 to prepare two electric school buses for service.
The latest large electric bus manufacturer on the scene is Chinese-based company BYD, which also offers a Type D and Type A models. On the small school bus front, Pegasus Specialty Vehicles goes into production with an electric Type A on a Zeus chassis the summer of 2022.
EPA Clean School Bus Program
The Bipartisan Infrastructure Law signed by President Joe Biden on Nov. 15, 2021 included the latest iteration of the Clean School Bus Program. It provides $5 billion over fiscal years 2022 through 2026, to fund the replacement of zero- and low-emission school buses.
Glossary of Green Bus Acronyms & Terms
- AC – Alternating Current
- BESB – Battery Electric School Bus
- BEV – Battery Electric Vehicle
- BMS – Battery Management System (Computer)
- CAGR – Compound Annual Growth Rate
- Capacity Fade – Reduced driving range. This is a function of time (Calendar Life) and is a function of time, temperature and SoC that is present even when the battery is not in use.
- CCS – Combined Charging System
- CE – Consumer Electronics
- CPM – Charging Point Manager
- CPO – Charging Point Owner
- C-rate – A measure of the rate at which a battery is charged and discharged relative to its maximum capacity
- Charging-as-a-Service – Subscription-based electric vehicle charging package that provides turnkey charging solutions with minimal upfront purchasing costs.
- DC – Direct Current
- DCFCs – Direct Current Fast Chargers
- DERs – Distributed Energy Resources
- DLM – Dynamic Load Management
- DoD – Depth of Discharge
- ESB – Electric School Bus
- ESG – Environment and Social Governance
- ESS – Energy Storage Sector
- EV – Electric Vehicle
- EVSE – Electric Vehicle Supply Equipment
- GHG – Greenhouse Gases
- HP – Horsepower (1 HP = 746 watts)
- ICE – Internal Combustion Engine
- IEM – Intelligent Energy Management
- Joule – Scientific unit for energy
- kW – Kilowatt (1,000 units of power)
- kWh – Kilowatt hour. The energy in a battery pack. Links power and energy through time.
- LCO – Lithium Cobalt Oxide Battery
- LEV – Low-Emission Vehicle – A standard mandating reductions in emissions of greenhouse gases and other air pollutants.
- LFP – Lithium Iron Phosphate Battery
- LMO – Lithium Manganese Oxide Battery
- LTO – Lithium Titanate Oxide Battery
- NCA – Lithium Nickel Cobalt Aluminum Oxide Battery
- NEV – New Energy Vehicle
- NMC – Lithium Nickel Manganese Cobalt Oxide Battery
- NOx – Oxides of nitrogen or nitrogen oxides.
- Power Fade – Reduced acceleration and gradeability. This is caused by cycling (charging and discharging) and temperature is involved.
- Scalability – When supply and demand come into balance
- SEI – Solid Electrolyte Interphase. Contributes to degradation of battery
- SOC – State of charge (SoC) is the level of charge relative to capacity (0% = empty; 100% = full).
- TCO – Total Cost of Ownership
- V2B – Vehicle to Building
- V2G – Vehicle to Grid
- V2X – Vehicle to Everything
- VPP – Virtual Power Plant
- Watt – A unit of Power. One watt = 1 Joule per second
- ZEV – Zero-Emission Vehicle. A standard requiring auto makers to sell increasing numbers of plug-in vehicles.
Idle Regulations By State
Browse regulations, laws and recommendations on school bus idling limits across the U.S. More information, research, recommendations and toolkits are available on the U.S. EPA’s National Clean School Bus web page (click on the “Idle Reduction” tab).
Alabama State Board of Education adopted a Resolution proclaiming the benefits of a No Idling Campaign for school buses. Alabama Department of Education, Pupil Transportation, No Idling Resolution.
None for school buses
Voluntary participation in School Bus Idling Reduction Program. School bus drivers must turn off the bus upon reaching a school or other location and must not turn on the engine until necessary to depart from the school or other location. Driver must park the bus at least 100 feet away from known and active school air intake system unless the school district has determined that alternative locations block traffic, impair student safety, or are not cost-effective. Employer of the school bus driver must ensure the driver is informed of the requirements of this policy. School district will post limited idling and idle reduction zone signs and alert bus drivers, parents, administrators, and vendors that engines should be turned off when a vehicle is waiting or parked. School districts will identify an indoor waiting area for individuals to discourage waiting for students in an idling vehicle; school districts should include “limited idling” policy in contracts with vendors and other vehicles that will be in close proximity to students. Complaints of noncompliance are reviewed, and remedial action is taken as necessary.
None for school buses
All school bus drivers must turn off bus upon stopping at a school or within 100 feet of a school and must not turn the bus on more than 30 seconds before beginning to depart from a school or from within 100 feet of a school. No idling allowed at any location greater than 100 feet from a school for more than five consecutive minutes, or a period or periods aggregating more than 5 minutes in any one hour.
Five-minute idle limit within a 60-minute period for all diesel-powered CMVs with a GVWR of more than 14,000 pounds, including school buses. Several local jurisdictions have idle limits ranging from 30 seconds to 15 for any diesel vehicle or certain vehicles weighing more than 10,000 pounds.
Source: Colorado House Bill 11-1275
Three-minute idle limit for all school buses except when stuck in traffic, experiencing “mechanical difficulties” or in process of loading/unloading students. Limit can be surpassed to operate heating, cooling or auxiliary equipment when the outdoor temperature is less than 20 degrees, “maintain safe temperature for students with special needs” or repair bus.
Source: Public Act No. 02-56
Five-minute idle limit for transit or school buses prior to passenger boarding or when passengers are on board. Idling for up to 15 consecutive minutes is allowed when the outside temperature is between -10 degrees and 32 degrees. No idling limit with the temperature is below -10 degrees.
District of Columbia:
Three-minute idle limit for all gasoline and diesel-powered vehicles, including school buses but exempting personal passenger vehicles. Idling can increase to five minutes when the temperature is 32 degrees or below.
No statewide regulation following 2011 repeal of law prohibiting idling of vehicles weighing 8,500 pounds or more. Tallahassee limits idling to five minutes for all city-owned or leased vehicles and equipment.
No idling zones at school campuses for morning drop off, afternoon pick up and at all destinations during field and athletic trips. Exceptions include loading or unloading a wheelchair, operating the air conditioning or heater for a medically fragile student. In cold weather situations, drivers may also congregate on one idling bus when waiting for students during afternoon pick up.
All motor vehicles, school buses not specifically mentioned. Engine may not be in operation while the motor vehicle is stationary at a loading zone, parking or servicing area, route terminal, or other off-street area.
Voluntary program from Idaho Department of Environmental Quality to erect Clean Air Zones near schools. Encourages school bus drivers to not idle on school grounds as well as other motorists to turn off engine while waiting for students.
State has several local ordinances restricting idling in diesel and commercial vehicles, but school buses are not specified. However, Chicago limits idling to three minutes in a 60-minute period for all city fleet vehicles. No idling signs are posted at schools in several communities in and around Chicago, and other cities have experimented with idle restriction policies.
“Schools shall adopt and enforce a written policy to address any idling vehicles on school grounds. This policy shall be modeled after the state department’s manual of best practices for managing IAQ [indoor air quality] in schools. This policy shall be available for the state inspector’s review.” The City of Fort Wayne restricts idling to five minutes per 60-minute period for all city-owned vehicles.
No known restrictions.
No statewide program, but the counties of Johnson and Wyandotte restrict idling to five minutes in a 60-minute period for all diesel-powered vehicles weighing more than 14,001 pounds that are designed primarily for transporting passengers on a public street or highway.
None for school buses
No statewide regulation, but New Orleans restricts all bus idling to 10 minutes in the French Quarter and the Garden District and to 20 minutes in the rest of the city.
Sources: New Orleans, Louisiana Code of Ordinances, Part II, Chapter 162, Vehicles for Hire, Article IX, Tour Vehicles and Buses, Section 162-941e; New Orleans, Louisiana Code of Ordinances, Part II, Chapter 162, Vehicles for Hire, Article IX, Tour Vehicles and Buses, Section 162-942; New Orleans, Louisiana Code of Ordinances, Part II, Chapter 122, Public Transit Vehicles, Section 122-52, Operating at Idle
“Passenger” buses may idle up to 15 minutes in a 60-minute period for a “to maintain passenger comfort while nondriver passengers are on board.”
All motor vehicles cannot idle for more than 5 consecutive limits, except Class L historic vehicles.
No “unnecessary” idling for all school buses and personal motor vehicles within 100 feet of school grounds. When outdoor temperature is less than 35 degrees or more than 80 degrees, school buses can idle for no more than three minutes in any 15-minute period to operate climate control when waiting to load or unload passengers. City-owned or leased vehicles in Boston must be inspected and certain models retrofitted with “effective emission-reduction equipment.”
No statewide regulation, but an advisory practice published by the Pupil Transportation Advisory Committee recommends keeping idling “to the minimum possible” and to turn off bus engines when waiting to load or unload students, except when operating lifts, “other specialized equipment” and when needing to warm the bus.
Source: School Bus Emissions/Idling
All school bus operators “must minimize to the extent practical” any engine idling and must park and load diesel buses “at sufficient distance from school air-intake systems.” Bus drivers must turn off diesel buses upon reaching a school or other destination and cannot turn the engine back on until it’s time to depart.
Source: Minnesota Statutes §123B.885
All school bus operators must minimize engine idling.
Source: Mississippi Code of 1972, Title 37 (Education), Chapter 11 (General Provisions Pertaining to Education), Miss. Code Ann. §37-11-71 (2014); Mississippi Department of Education, Office of Healthy Schools
Missouri: The Missouri Department of Natural Resources encourages school districts to adopt anti-idling policies for school buses. Several counties including St. Louis and the Kansas City Ozone Maintenance Area restrict bus idling to 15 minutes in any 60-minute period to maintain passenger comfort.
The Montana Department of Environmental Quality encourages schools to establish guidelines to reduce or eliminate idling of buses and other vehicles and to park buses away from intake vents and children’s gathering places.
No program for school buses
All diesel trucks and buses statewide cannot idle for more than 15 consecutive minutes.
Sources: Nevada Administrative Code (NAC) 445B.576; Clark County Air Quality Regulations, Section 45; Washoe County, District Board of Health Regulations Governing Air Quality Management, Section 040.200
All diesel and gasoline-powered motor vehicles statewide cannot idle for more than five consecutive minutes when the temperature is above 32 degrees or for more than 15 consecutive minutes when the temperature is between -10 and 32 degrees. There is no idling restriction when the temperature falls below -10 degrees.
Idle restriction of three consecutive minutes for diesel- or gasoline-powered school buses, or 15 consecutive minutes in a 60-minute period when loading or unloading passengers, except when waiting in traffic or operating passenger compartment heaters or air conditioners.
No known restrictions.
Diesel- or nondiesel-powered, heavy-duty vehicles “designed primarily to transport persons or property” can idle a maximum of five minutes statewide. New York City reduces that limit to three minutes or one minute if the vehicle is adjacent to a school. Several counties have their own local regulations.
State law prohibits all unnecessary school bus idling on school grounds and prohibits the warming up of buses for more than five minutes.
West Fargo has only known regulation, a five-minute limit for all school district-owned or contracted school buses when the temperature is higher than 32 degrees. The limit is increased to 10 minutes if the temperatures is between freezing and 10 below and to 15 minutes if the temperature falls below -10.
Source: West Fargo School District 6 School Bus Anti-Idling Program
No state regulation is found, but several cities limit or prohibit idling of city or municipal vehicles, however, school buses are not specifically mentioned. Participating cities include Cincinnati, Cleveland and Columbus.
Source: Alternative Fuels Data Center
No regulations, but the Association of Central Oklahoma Governments recommends school bus idling of no more than five minutes as well as the implementation of local driver training programs and written policies for exact idle times for ambient temperature ranges.
Source: Alternative Fuels Data Center
Five-minute idle limit in any 60-minute period “on any premises open to the public.” Applies to commercial vehicles with a GVWR of more than 10,000 pounds. Idling allowed for up to 30 minutes when waiting to load or unload.
Sources: Chapter 349, Oregon Laws, 2011
School buses may not idle more than 15 minutes in a continuous 60-minute period, unless the outside temperature is less than 40ºF, then idling is allowed for a period or periods aggregating not more than 20 minutes in any 60 minute period.
Five minutes in any 60-minute period for all diesel motor vehicles, excluding those with auxiliary power units. Allowable 15-minute idle per hour to provide heat when the ambient temperature is between zero and 32 degrees. No restriction when the temperature falls below zero.
Sources: Rhode Island General Laws § 31-16.1-3 and § 23-23-29; State of Rhode Island and Providence Plantations Department of Environmental Management Office of Air Resources Air Pollution Control Regulation, No. 45.
All 10,000-plus pound, commercial diesel buses designed to carry 16 or more passengers may idle no more than 15 minutes in any 60-minute period to operate heaters or air conditioners.
No known restrictions.
The City of Chattanooga restricts idling to five minutes for any vehicle powered by a diesel engine.
Vehicles with GVWR of more than 14,000 pounds operating in North Central or Central Texas and have signed a memorandum of agreement with the Texas Commission on Environmental Quality must restrict idling to no more than five consecutive minutes, or 30 minutes for passenger comfort and safety “in vehicles intended for commercial or public passenger transportation.” A sample police to limit school bus idling for school districts and charter schools recommends no idling while parked at a school or school event, or waiting for students during field trips, extracurricular activities or other events where students are transported off school grounds. Exceptions include minimum idling to heat or cool the bus before departure; turbo-charged diesel engine cool down or warm up, based on manufacturer specifications; limited idling during emergency situations; use of heaters or air conditioners during loading, unloading and transport of students with special needs; and use of bus headlights and four-way flasher warning lights for visibility purposes.
School bus drivers arriving at loading or unloading areas to drop off or pick up passengers shall turn off their buses as soon as possible to eliminate idling time and reduce harmful emissions. The school bus shall not be restarted until it is time to depart and there is a clear path to exit the pick-up area. At school bus depots, limit the idling time during early morning pre-trip and warm-up to what is recommended by the manufacturer (generally 3-5 minutes) in all but the coldest weather; buses shall not be idled while waiting for students during a field trip, extracurricular activities, or other events where students are transported off school grounds. In colder weather, schools are directed, where possible, to provide a space inside the school where bus drivers who arrive early can wait; in colder weather, if the warmth of the bus is an issue, idling is to be kept to the very minimum and occur outside the school zone. The “warmed” bus is to enter the school zone as close to pick-up time as possible to maintain warmth, and then shut down. Transportation Operations staff are directed to revise bus schedules so that school bus caravanning can be avoided and the cleanest buses are assigned to the longest routes. All drivers shall receive a copy of this standard at the beginning of every school year. The cool-down needed for the turbo must be addressed in a way that will protect the equipment from damage. Where possible, a slow, idled-down approach to the loading zone should be used to provide the cool-down needed for the turbo. Where sufficient idle-down is not obtained in the approach to the loading zone, a maximum of 3 minutes of idle-down is permissible. Each driver shall receive a minimum of 30 minutes of idling reduction instruction.
School buses on school grounds may idle no more than five minutes in a 60-minute period, except: to operate special equipment for disabled persons; to operate safety equipment other than lighting systems; and when the vehicle is being serviced and engine operation is essential to function being performed.
Propulsion engines of motor vehicles licensed for commercial or public service may idle no more than 3 minutes in a commercial or residential area. Exceptions include: propulsion engines providing auxiliary power for purposes other than heating or air conditioning; tour buses may idle during hot weather for up to 10 minutes in order to maintain power to the air conditioning system; diesel-powered vehicles may idle for up to 10 minutes to minimize restart problems. The cities of Williamsburg and Richmond enforce five-minute idling restrictions for all city vehicles except under extreme weather conditions.
Sources: Virginia Administrative Code, Title 9, 5-40-5670; Personnel Manual of the City of Williamsburg, Section 602; Anti-Idling Policy: Vehicles and Equipment: A.R. 6.6, Issued by the Office of the Mayor
No application to school buses in state code, but the state’s School Bus Driver Handbook contains a section (page 21) on Diesel Emissions and Anti-Idling Policies. It recommends that bus drivers should turn off engines upon reaching the school or as soon as engine specifications permit.· If severe climate conditions require idling, drivers should idle buses off school grounds and only as long as necessary. Drivers can complete most of their pre-trip inspection without the engine running. The pre-trip time with the engine running should take no more than five minutes and will allow drivers to complete the electrical portion of the inspection and any brake check. Air-brake equipped school buses need only be run long enough to build air pressure prior to departure from the bus garage (three to five minutes at most). School buses with hydraulic braking systems need no more than a 30-second warm-up. Time must be allowed for inspection of necessary items.
Source: Revised Code of Washington 46.61.600; www.k12.wa.us/Transportation/pubdocs/SBDhandbook.pdf
Fifteen-minute restriction per every 60 minutes for diesel-powered commercial motor vehicles with a gross weight of 10,000 pounds or more. Exceptions include “buses providing heating and air conditioning for nondriver passengers” and “School buses idling off school grounds queuing for discharge or pickup of students.”
The City of Milwaukee requires school bus drivers to turn off engines while on Milwaukee Public School premises. Idling for early-morning warm-up must be limited to manufacturers’ recommendations. The City of Madison restricts bus idling to 15 minutes except when the outside temperature is below 40 degrees or above 80 degrees. Clean Air Wisconsin also recommends all school districts should enforce limits by posting signs and notifying contractor companies they work with; bus drivers should not idle for “lengthy times” in the morning and when waiting for students outside of schools, should arrive closer to the actual pick-up time or ask to wait in school lobby if it is too cold or hot to wait on the bus; and fleets should educate their drivers on best practices via training sessions.
Sources: Milwaukee Board of School Directors Busing Contract; City of Madison, Code of Ordinances 12.129(2); www.cleanairwisconsin.org/diesel.php
No unattended standing” allowed for “vehicles on roads outside of business or residence districts.” No specific mention of school buses. But the Department of Education recommends school districts engage in no unnecessary idling.
Fuel & Environmental Web Resource Links
- Alternative Fuels and Advanced Vehicle Data Center
- Energy Smart Schools
- Gas Technology Institute
- National Biodiesel Board
- National Renewable Energy Laboratory
- Natural Gas Vehicle for America
- Natural Gas Vehicle Institute
- Propane Education & Research Council
- Solar Action Alliance
- U.S. Department of Energy Clean Cities Program
- Association of Diesel Specialists
- Diesel Technology Forum
- Energy Information Administration: On-Highway Retail Diesel Prices
- Engine Manufacturers Association
- American Council of Science and Health
- California Air Resources Board
- California’s Diesel Risk Reduction Program
- California Energy Commission
- EPA: Heavy-Duty Diesel Fuel Analysis Program
- Emission Control Potential for Heavy-Duty Diesel Engines
- Environment and Human Health
- National Association of Fleet Administrators: Sustainability & Alternative Fuels
- Natural Resources Defense Council
- South Coast Air Quality Management District
- Union of Concerned Scientists
- S. Environmental Protection Agency
- S. General Accounting Office
- Scientific Review Panel on Toxic Air Contaminants