The Transportation Department has added its voice to the ongoing debate over how to reduce greenhouse gas emissions, offering an analysis of a half-dozen strategies that are already under way or under consideration.

Transportation is a major contributor to the greenhouse gases that most scientists say exacerbate global warming. Tailpipe emissions contribute almost 30 percent of greenhouse gases

, DOT said in a report to Congress published in April. Most of those emissions, almost 80 percent, come from cars. Trucks contribute most of the remainder, 19 percent.

Strategies For Reducing GHGs

The 605-page report does not make any recommendations. Instead it looks at the policy options that arise from a number of strategies familiar to trucking.

One is more use of low-carbon fuels such as biodiesel. Another is improving fuel economy. On that score DOT is working on a comprehensive analysis of how it might set fuel economy standards for the trucking industry (See story).

DOT also looked at improving the efficiency of the transportation system overall. This would include such steps as lowering speed limits and reducing congestion at bottlenecks by adding capacity and traffic management.

A fourth strategy would be to "reduce carbon-intensive travel." This might be achieved by raising fuel taxes, charging drivers by the mile and expanding transit services.

Another strategy is to improve transportation planning and investment, DOT said. This could be done by supporting mixed-use development in which homes and businesses are close to each other, and by supporting options such as transit, biking and walking as an alternative to driving to work and for errands. Here DOT mentions the idea of promoting lower-carbon freight options such as rails and marine transport over trucks.

The sixth strategy, which is now being set forth in Congress in the cap-and-trade debate, is to increase the cost of carbon energy.

Trucking's Contribution to Emissions

Of particular interest to trucking, DOT's analysis finds that due to deregulation and the emergence of just-in-time inventory management, among other factors, the industry's contribution to greenhouse gas emissions has increased dramatically over the past two decades.

Since 1990, greenhouse gas emissions from medium and heavy trucks have increased 77 percent, DOT said. This is due to what DOT terms "decreasing fuel efficiency," as measured by ton-miles carried, and increasing demand for truck services. Between 1990 and 2005, carbon dioxide emissions per ton-mile increased almost 13 percent, while ton-miles grew 58 percent.

"These changes were driven by an expansion of freight trucking after economic deregulation of the trucking industry in the 1980s; widespread adoption of just-in-time manufacturing and retailing practices (that increased) highway congestion; and structural changes in the economy that produce higher-value, lower-weight, and more time-sensitive shipments better served by trucking," DOT said.

Richard Moskowitz, vice president and regulatory affairs counsel at American Trucking Associations, pointed out that ton-miles may not be an appropriate way to measure truck emissions.

"A truckload of potato chips won't weigh as much as a truckload of batteries," he said.

He also noted that federal clean air standards have in fact led to a reduction in truck fuel efficiency. "For example, the move to exhaust gas recirculation in 2002 produced about an 8 percent penalty. The 2006 transition to ultra low sulfur diesel resulted in a 1 percent penalty and in 2007 particulate traps further eroded fuel efficiency. In addition, these emissions standards increased the vehicle weight."

To see the full report, click here.