Two approaches to green transportation

Transportation accounts for 28 percent of U.S. greenhouse gas emissions and about a quarter of global greenhouse gas emissions from energy use. Much of your transportation impact is invisible, going beyond the miles you travel to work or fly home for Thanksgiving.

Think of the shipping for the last thing you bought online, or the distance your groceries travel to get to the supermarket. In 2005, an average of 68 tons of freight moved 15,310 miles for each U.S. citizen. That's akin to dragging a fully loaded 737 from New York to L.A. five times.

RMI is a chief proponent of improving the efficiency of our cars, trucks, and airplanes. Our car work, for instance, focuses on lightweighting to increase vehicle miles-per-gallon. But radically reducing transportation's impact won't happen without addressing the other part of the equation: how many miles we (or the products we consume) travel. 

To illustrate that point, here are a few examples:

Mexico City's Metrobus
To reduce urban VMT, the World Resource Institute's EMBARQ team launched a project with Mexico City to address chronic congestion, pollution, and safety problems on city streets.

In June 2005, the city launched a rapid transit bus system, called Metrobus, along the one of the city's most congested arteries, Insurgentes Avenue. In its first year, the bus carried more than 100 million passengers, drastically cutting vehicle miles traveled in the city. Along with this success, the project is making each of those miles as clean and efficient as possible by testing and integrating compressed natural gas, hybrid buses, and low-sulfur diesel buses.

How successful has the program been? In its first year, Metrobus cut an estimated 36,000 tons of greenhouse emissions from the city's smog-filled air and reduced passengers' exposure to toxic pollutants by half. For comparison's sake, the U.S. emits about 26.5 tons of CO2 equivalents per citizen. Metrobus also cuts on average 30 minutes off most people's commute and has inspired several other Mexican and global cities to try similar systems. 

Plug-in cars, shorter commutes
Vehicle-to-Grid technology (V2G), a Rocky Mountain Institute favorite, focuses on improving vehicle efficiency. V2G refers to plug-in hybrids that draw and store energy from the electric grid for driving but give energy back to the grid when parked. 

The efficiency gains from plug-in hybrids are two-fold. Powering a car with electricity releases fewer greenhouse gases than powering a car with petroleum. RMI work and other reports on plug-in hybrids have confirmed this.

By giving electricity back to the grid when parked, plug-in hybrids can reduce the demand on electricity providers during peak demand hours when the efficiency of producing electricity is at its worst, but few people are driving—like three in the afternoon on a hot summer's day. 

But V2G also has to take into account miles traveled. In order for V2G to reduce greenhouse gas emissions effectively, cars have to stay in electric mode as long as possible—about 40 miles with current technology, though some new models boast over 100 miles between charges. 

That means any successful V2G strategy depends on shortening commutes—or, put differently, reducing vehicle miles traveled so that most trips can be all-electric.

VMT and MPG for the products we buy
Sometimes, VMT and MPG solutions can seem to be at loggerheads. Take, for example, the concept of "food-miles." Eating locally is popular with people who want to lead more green lifestyles. But some studies challenge the notion that the food-miles (the VMT portion for your food) is the key to evaluating that meal's environmental impact, because some small, local purveyors often use inefficient modes of transportation and processing (the MPG portion). 

A study from New Zealand found that New Zealand lamb, eaten in London, had a smaller greenhouse gas footprint than English lamb, because growing and processing conditions in New Zealand are cleaner, and because transportation by ship has a much smaller impact than truck.

Making sense of it all
The jury is still out on the whether food miles are a good measure of a product's "greenness." The balance between food-miles and production and processing efficiency can vary greatly on the type of food—and the benefits of local food may go far deeper than greenhouse gas accounting. But the debate does shed light on the importance of considering both VMT and MPG when trying to live more sustainably. 

Improving efficiency through technologies like lightweight materials and hybrids is a smart and profitable thing for us to do. But it's no panacea for solving the impact transportation has on the environment. Urban planning, government policy, corporate commitment, and individual choice will all play roles. 

The first step for everyone, from high-level policy makers to the consumer, is to recognize that a sustainable path for transportation will require changes in technology and behavior—in other words, MPG and VMT will have to go hand-in-hand. 

Laura Shewel is an analyst with MOVE – The Transportation Innovation Group at Rocky Mountain Institute.