On the heels of mandating new fuel efficiency levels for cars and light trucks by 2025, the Obama administration has announced fuel efficiency standards for heavy-duty trucks and buses . The United States becomes the second nation, after Japan, to put such rules into place. Heavy-duty vehicles emit 20 percent of U.S. transportation greenhouse gas emissions, which contribute to climate change. The new efficiency rules will curtail these emissions, reduce the nation's oil use -- of which the transportation sector is responsible for 70 percent -- and provide significant economic benefits.

Even prior to the latest rules, the United States had taken important steps to limit the environmental impact of on-road diesel engines. In 2000, the Environmental Protection Agency (EPA) mandated significant improvements in diesel fuel quality and tailpipe emissions standards for nitrogen oxides (NOx), sulfur oxides (SOx), and particulate matter. As a result, diesel vehicles built after 2006 produce less than 10 percent of these emissions than similar vehicles built 15 years prior. Locomotives are subject to similar restrictions. These policies severely mitigate diesel vehicles’ negative impact on air quality and public health.

The new fuel efficiency standards, to be phased in from model years 2014 to 2018, will provide direct economic and climate benefits. Under the new standard, vehicles built in this period are projected to use 530 million fewer barrels of oil over their lifetimes, saving an EPA-estimated $50 billion. These savings will dwarf the added cost of engine improvements: a single large truck is expected to net $73,000 in lifetime fuel savings under the new standard.

In respect to climate change, the efficiency standard is expected to reduce greenhouse gas emissions by 270 million tons over the lifetime of vehicles built during the phase-in period. This savings, spread out over more than a decade, is equal to approximately 15 percent of the United States’ current single-year emissions from on-road transportation.

Large semi-trucks built in 2017 will be required to be approximately 20 percent more fuel efficient than today’s vehicles. Currently, unregulated trucks achieve an average of six miles per gallon. The new standard will therefore improve semi-truck fuel economy by slightly more than one mile per gallon. For every 100 miles traveled, one truck would save three to four gallons of fuel.

Heavy-duty pickup trucks and vans must improve fuel efficiency by approximately 15 percent for model year 2018. Buses, delivery vehicles, garbage trucks, and other vocational vehicles must meet a 10 percent increase in the same period. Regulators expect these improvements to save one gallon for every 100 miles driven by these vehicles.

President Obama noted that the regulations have the full support of the trucking industry . The new standards were in fact spurred by requests from manufacturers and users of heavy-duty vehicles, whose budgets have been squeezed by rising fuel costs.

The consensus-driven standards are an important step forward to reducing oil use by heavy-duty vehicles, and further reductions can be made by implementing higher efficiency standards past 2018 and through other complementary measures.

Shifting to alternatives to diesel fuel, including biodiesel, natural gas , and electricity in select cases can reduce oil demand. These alternatives have all been successfully deployed at relatively small scales and are very promising, but face questions regarding scale-up potential and economic and environmental costs.

Significant energy savings would be possible through changes to the nation’s freight system. This could be achieved in part through greater reliance on rail to move goods over long distances, freeing large trucks to make shorter trips from rail hubs to end users.

And engine idling is a commonly overlooked source of heavy-duty engine inefficiency. Idling of large vehicles is estimated to consume over one billion gallons of fuel each year . Long-haul trucks often idle for hours at rest stops to provide drivers with heat and electricity. Behavior change and fines can partially curb idling, but on-board generators and other user-friendly new technologies provide an exciting opportunity for substantive idling reductions.