Conventional wisdom suggests that the most effective way to reduce light-duty vehicle emissions in metro areas is by reducing car travel. Many planners and policy makers have embarked on a policy of removing freeways, adding road diets, installing speed humps and making life miserable for commuters. However, conventional wisdom is completely wrong.
Much of the emissions in large metro areas come from carbon dioxide and volatile organic compounds. And most of those emissions occur when cars are traveling in stop and go traffic rather than free-flow speeds. Barth, a professor of Electrical Engineering and Boriboonsomsin, an Environmental Research Scientist at the University of California Riverside found that for Carbon Dioxide and Nitrogen Oxides, emissions versus speed is a U-shaped pattern where cars traveling at free-flow speeds (between 40-70 miles per hour) release less carbon dioxide than cars traveling in stop and go patterns (speeds between 0 and 30 miles per hour). In other words, eliminating stop and go traffic and severe congestion improves the environment far more than restricting car travel. In fact study authors found that with adopted tighter vehicle fuel efficiency requirements and engine technology, increasing free flow travel speeds to 40 miles per hour is by far the most effective way to reduce emissions in the light duty vehicle fleet. And most major metro areas face numerous corridors with congested traffic from six to twelve hours per day.
The most effective way to add this needed capacity is to add variably priced express lanes on freeways which provide an option for the driver (the pricing is dependent on congestion to keep traffic moving at 45 miles per hour or higher). For busy arterials the principal is the same. Commuters could use variably priced bridges to keep traffic moving at 35 miles per hour or higher. On both types of roads, the priced lanes are completely optional. The priced lanes would be new; commuters would always have the option of using the existing free lanes.
In addition to having the users pay for a large part of the improvement, this policy would increase speed and decrease congestion. This pricing also encourages people to travel only as needed, reducing induced demand (the tendency of new roads to generate new car trips), which is the reason most planners dislike freeway and arterial improvements. Such improvements would also improve the fuel efficiency of buses and improve transit service. If the goal is to improve the environment, speeding up travel by adding lanes and pricing those lanes is the most effective solution.