Increased Intersection Capacity & Efficiency
Under many traffic conditions, an unsignalized roundabout can operate with less delay to users than traffic signal control or all-way stop control.
Unlike all-way stop intersections, a roundabout does not require a complete stop by all entering vehicles, which reduces both individual delay and delays resulting from vehicle queues. Many drivers adjust their speed to take advantage of approaching gaps in circulating traffic, particularly in the off-peak period. If there is no traffic in the roundabout, they don't have to stop at all.
A roundabout can also operate more efficiently than a signalized intersection because drivers are able to enter from different approaches at the same time when traffic is clear without the delay incurred while waiting for the traffic signal to change (i.e. no yellow or red times).
Preliminary results from ongoing studies at Kansas State University (KSU) on several modern roundabouts are showing that modern roundabouts have greater overall operational efficiency than all other forms of traffic control. In one example, a KSU study analyzed changes in before and after delay and percent stopping.
|Measure of Effectiveness||Before||After||%Diff||Average Operational Performance|
|Source: Study of Operational Performance and Environmental Impacts of Modern Roundabouts in Kansas|
|Average Intersection Delay (Seconds/Veh)||20||8||-65%||Average vehicle delay for all vehicles entering the intersection|
|Max Approach Delay (Seconds/Veh)||34||10||-71%||Average vehicle delay for the approach with highest delay|
|95% Queue Length (Feet)||190||104||-53%||Value below which 95% of all observed lengths of vehicles being backed fall|
|Proportion Stopped–Intersection (%)||58||29||-52%||Proportion of vehicles approaching intersection and required to stop|
|Max Prop. Stopped (%)||62||37||-62%||Highest proportion of vehicles stopped on one approach|
|Degree Of Saturation–Intersection (v/c)||0.463||0.223||-53%||Measure of congestion on the roadway that is being used by traffic|
All of this data boils down to less cars stopping, fewer automobiles idling, and more vehicles moving towards their intended destinations. Consider reduction in person-hours of delay and reduced vehicle delay throughout the day (not just peak hours).
It adds up...
A roundabout serving 15,000 vehicles a day saves, annually, a total of 15,000 gallons of fuel, compared to the same intersection controlled by a traffic light, according to analysis from the Washington State Department of Transportation.
A key finding of the Insurance Institute for Highway Safety in one study is that vehicle delays at the 10 intersections would have been reduced by 62-74 percent, saving 325,000 hours of motorists' time annually. Fuel consumption would have gone down by about 235,000 gallons per year, and there would have been commensurate reductions in vehicle emissions. Assuming $18 per hour in worker wages and $3.50 per gallon of gas is a community cost of $6.7 million.