When designed correctly modern roundabouts can significantly reduce the amount of pollutants released into the atmosphere from idling vehicles when compared to other intersections. At signalized intersection locations, where actuated signals are not present, many drivers must wait up to 20 seconds or longer for the light to turn green, even during off-peak hours when no crossroad or opposing traffic is present on other approaches. Since the entry control at modern roundabouts utilizes the “yield to left” principle, drivers spend less time idling their vehicles prior to entering the intersection.
When a vehicle is stopped at an intersection, the vehicle’s exhaust emits undesirable gases into the atmosphere such as carbon monoxide and nitrogen oxides. These emissions in large quantities contribute to unclean air and smog, both of which can be harmful to the public and the environment. Less time idling at an intersection means fewer pollutants are emitted into the atmosphere and fewer fuel resources are consumed. A study conducted by the Vermont Department of Public Service titled “Modern Roundabouts, Global Warming, and Emissions Reductions: Status of Research, and Opportunities for North America”, supports these findings. The study’s executive summary states:
“Two decades of intersection control modeling and software development and research, establish that substantial fuel savings at busy intersections can be gained by employing roundabouts rather than traffic signals. Reduced fuel consumption, pollution emissions and GHGs (green house gases) are demonstrated through analysis of empirical data and modeling reported from existing US roundabouts and those under development.”
At many urban intersections where traffic signals are present, traffic queuing can add long delays to a trip. Intersections as shown on the left contribute to large quantities of emissions released into the atmosphere. It is these emissions that contribute to unclean air and smog, both of which can be harmful to the environment and people with respiratory illnesses.
There are limited international and U.S. studies available that quantify exact reductions in emissions modern roundabouts hold over conventional intersections. The data and conclusions from these studies vary depending upon existing conditions and other factors such as existing traffic volumes, traffic patterns during peak hours of the day, and entry control (stop signs or traffic signals).
A recent study was conducted by Kansas State University (Environmental Impacts of Kansas Roundabouts, September 2003) at three different locations that were converted from four-way stop control intersections to modern roundabouts. The report found a 38-45 percent decrease in Carbon Monoxide emissions, a 55-61 percent decrease in Carbon Dioxide emissions, a 44-51 percent decrease in Nitrogen Oxides, and a 62-68 percent decrease in Hydrocarbons. Other compiled studies found that when conventional intersections (signalized and unsignalized) are converted to modern roundabouts, there is an average reduction of 30 percent in carbon monoxide and nitrogen oxides, and a 30 percent reduction in fuel consumption. These preliminary conclusions indicate that modern roundabouts significantly reduce the amount of pollutants released into the atmosphere and reduce overall fuel consumption.
A Status Report published by the Insurance Institute for Highway Safety in the fall of 2005 (Volume 40, No. 9) studied 10 intersections where roundabouts were considered as alternatives for an intersection improvement project, but ultimately the road authority determined to use traffic signals as the entry control. During this study, researchers estimated vehicle delays and fuel consumption at the existing conventional signalized intersections, and compared them with estimates of what could have been expected if a modern roundabout were chosen as the preferred alternative at the ten intersections.
A key finding from the study indicated that combined vehicle delays at the 10 intersections would have been reduced by 62-74 percent, saving 325,000 hours (or 37.10 years) of motorist’s time annually. It was estimated that fuel consumption would have also decreased by about 235,000 gallons per year. Assuming an average cost of $2.50 for a gallon regular gasoline, that is an annual savings of $587,500 for the ten intersections. Since less fuel would have been consumed at modern roundabout intersections, fewer emissions would have been released into the atmosphere.
Gas prices in many parts of the country are on the rise, especially during peak travel times of the year. Converting more intersections to modern roundabouts can reduce overall fuel consumption.