In a paper published in Environmental Research Letters (ERL), Jalel Sager and colleagues from the University of California show that to meet greenhouse-gas emission and climate-reduction goals for the year 2050, the way in which we use LDVs has to change.

Co-author Daniel Kammen told environmentalresearchweb: "Reducing LDV emissions is often thought of as a technological challenge, with efforts going into the development of more efficient cars or fuels that produce fewer greenhouse gases per unit energy. However, by decomposing transport-sector emissions into technological and behavioural drivers, we show that even significant technological advances will be insufficient to meet climate goals, unless the growth in LDV use slows or reverses."

To quantify the carbon dioxide mitigation challenge for the transport sector, the researchers surveyed 2007 LDV usage and fuel economy in an economically diverse set of countries. They found that the large differences in per capita LDV greenhouse-gas emissions (range: ˜100–4000 kg of carbon dioxide equivalent per year) are principally explained by differing national per_capita LDV use (range: 300–13,000 vehicle kilometres travelled (VKT) per year), rather than to fleet average fuel-efficiency and carbon-intensity factors, which reflect the broadly similar car technology worldwide.

The researchers forecast that meeting greenhouse-gas targets through technology developments alone would require universal deployment of one or more of the following clusters: electric vehicles running on nearly zero-carbon electricity, cellulosic biofuel-powered vehicles achieving 300 miles per gallon (0.78 l per 100 km), or gasoline-fuelled vehicles achieving in excess of 1000 mpg (0.24 l per 100 km).

"These performance levels exceed even the most optimistic technology scenarios for the year 2050," said Kammen. "This shows that reducing greenhouse gases emitted by LDVs is a behavioural issue, not a technological one."

Kammen cites several success stories of cities that have relatively low greenhouse-gas emissions from LDVs because of relatively compact urban development. For example, citizens of Hong Kong, Seville, Turin, Valencia, Lisbon, Bologna, and Moscow use between 5,000 and 11,000 MJ per capita per year for travel through these relatively compact areas, with more than half of all trips taken by foot, bicycle or public transport. Meanwhile, in cities with higher personal vehicle use, such as Chicago, Houston, San Diego or Washington, inhabitants use 44,000 to 86,000 MJ per capita per year, with less than 16% of all journeys accomplished through non-motorized or public means.

As well as improved urban planning and public transport, the researchers say that pricing policies and parking and congestion fees have also been shown to influence travel behaviour. Steadily increasing fuel taxes have proven especially useful in many developed countries, for example Germany, in reducing VKT and encouraging automakers to increase fuel efficiency over time. They point out that "the US, with some of the lowest fuel taxes in the developed world, seems ripe for such a measure".

"There are so many opportunities available to us to reduce our greenhouse gases from LDVs," said Kammen. "The question is, can we implement them quickly enough?"