Engineers at the Univeristy of California at Riverside have found some hard evidence that traffic congestion is terrible for greenhouse gas emmissions. By slowing traffic below their optimal operations level, it boosts CO2 emissions. The same thing happens, however, when cars and trucks drive faster their their engineered efficiency levels, which tend to be upwards of 70 mph. The key is to keep traffic moving at free flow between about 30 and 60 mph.
Here's the abstract to "Real World CO2 Impacts of Traffic Congestion," a paper presented by Matthew Bart and Kanok Boriboonsomsin at the Transportation Research Board Meetings last January in Washington, D.C.:
Transportation plays a significant role in carbon dioxide (CO2) emissions, accounting for approximately a third of the United States' inventory. In order to reduce CO2 emissions in the future, transportation policy makers are looking to make vehicles more efficient and increasing the use of carbon-neutral alternative fuels. In addition, CO2 emissions can be lowered by improving traffic operations, specifically through the reduction of traffic congestion. This paper examines traffic congestion and its impact on CO2 emissions using detailed energy and emission models and linking them to real-world driving patterns and traffic conditions. Using a typical traffic condition in Southern California as example, it has been found that CO2 emissions can be reduced by up to almost 20% through three different strategies: 1) congestion mitigation strategies that reduce severe congestion, allowing traffic to flow at better speeds; 2) speed management techniques that reduce excessively high free-flow speeds to more moderate conditions; and 3) shock wave suppression techniques that eliminate the acceleration/deceleration events associated with stop-and-go traffic that exists during congested conditions.