Many cities have begun to use modern roundabouts - so designated to specify those with adequate signage and features such as mid road islands to allow safe road crossing by pedestrians and cyclists. Vermont is one of the leading American states in their use- as demonstrated in this paper by Tony Redington - MODERN ROUNDABOUTS, GLOBAL WARMING, AND EMISSIONS REDUCTIONS: STATUS OF RESEARCH, AND OPPORTUNITIES FOR NORTH AMERICA - which states the following:
"It is suggested that 25 roundabouts replacing existing traffic signals in the City of Burlington, Vermont would equate to over 20% of that City’s goal of bringing GHG emissions to 10% below the base line 1990 level."
"The modern roundabout era began in 1996 with the adoption by Britain of “yield-at-entry” rule for vehicles entering a roundabout, giving vehicles in the circular travelway the right-of-way for the first time."
"Taking an arbitrary overall delay figure midway between the a.m. and p.m. delay–26.5 seconds delay for the 28,000 average daily traffic translates to a reduction of 75,231 hours yearly of stop delay..To translate stop delay to fuel usage... the 75,231 hours of stop delay translates to an annual motor fuel consumption decrease of 30,845 gallons."
A third reference Modern Roundabouts and the Environment provides some further statistics on emisisons reductions, based on experience in Kansas with roundabouts:
"The report (Environmental Impacts of Kansas Roundabouts, September 2003) 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."
Here is a Roundabout list for the cities of Ottawa-Gatineau.
Finally, safety advantages are addressed in this report from Michigan Safety Benefits of Modern Roundabouts which states: "For vehicle to pedestrian collisions, the number of conflicts at the intersection is reduced from 24 points with the conventional signalized intersection to 8 points with the modern roundabout, or a 67 percent reduction."
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