Reduction in number and severity of crashes
Several features of the modern roundabout promote safety. The safety benefits alone should be enough to convince roundabouts are a great idea. According to a study by the Institute for Highway Safety of locations where stop signs or traffic lights were replaced by roundabouts, all crashes were reduced by 37 percent and serious crashes fell by 75 percent, including a 90 percent reduction in fatalities. This means not only fewer crashes, but also most will be able to walk away from them.
Source: FHWA and IIHS
At traditional intersections, some of the most common types of crashes are right-angle, left-turn, and head-on collisions. These types of collisions can be severe because vehicles may be traveling through the intersection at high speeds to "beat the red light." With roundabouts, these types of potentially serious crashes essentially are eliminated because vehicles travel in the same direction.
40th and Sheridan - Before
40th and Sheridan - After
Installing roundabouts in place of traffic signals can also reduce the likelihood of rear-end crashes by reducing abrupt stops at red lights. The vehicle-to-vehicle conflicts that occur at roundabouts generally involve a vehicle merging into the circular roadway, with both vehicles traveling at low speeds – generally less than 25 mph in urban areas.
A traditional intersection has 32 potential conflict points
A roundabout has only 8 potential conflict points
Pedestrians need only cross one direction of traffic at a time at each approach as they traverse roundabouts, as compared with unsignalized intersections. In addition, crossing distances are relatively short, and traffic speeds are lower than at traditional intersections. The conflict locations between vehicles and pedestrians are reduced from 16 to 8 since conflicting vehicles come from a more defined path at roundabouts. In addition, the speeds of motorists entering and exiting a roundabout are reduced with good design.
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 vehicle yields before entering roundabout. If the path is clear, there is no need to stop.
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
||Average Operational Performance
|Average Intersection Delay (Seconds/Veh)
||Average vehicle delay for all vehicles entering the intersection
|Max Approach Delay (Seconds/Veh)
||Average vehicle delay for the approach with highest delay
|95% Queue Length (Feet)
||Value below which 95% of all observed lengths of vehicles being backed fall
|Proportion Stopped–Intersection (%)
||Proportion of vehicles approaching intersection and required to stop
|Max Prop. Stopped (%)
||Highest proportion of vehicles stopped on one approach
|Degree Of Saturation–Intersection (v/c)
||Measure of congestion on the roadway that is being used by traffic
|Source: Study of Operational Performance and Environmental Impacts of Modern Roundabouts in Kansas
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.
Projected Traffic Comparison Simulation - Year 2025
By reducing vehicular delay through increased capacity and efficiency, the community will improve air quality through lowered emissions.
Driving through a typical intersection usually includes a complete stop, idling, then acceleration to proceed through the intersection.
With a roundabout, the stop may be eliminated, which reduces the fuel needed to re-accelerate.
The potential reduction in fuel consumption can increase in air quality. A KSU study found a significant reduction in emissions in before and after measurements.
|Measures of Effectiveness
|Carbon Monoxide (CO) Kg/hr
|Carbon dioxide (CO2) Kg/hr
|Oxides of Nitrogen (NOx) Kg/hr
|Hydrocarbons (HC) Kg/hr
|Source: Impact of Modern Roundabouts on Vehicular Emissions
The US Department of Transportation's Congestion Mitigation and Air Quality (CMAQ) Improvement Program provides funding for transportation projects aimed at reducing emissions levels. If a community is in air quality attainment, then those funds may be used for any project; if not, then those funds and others are directed to only projects related to bettering the air quality. Roundabouts help the community retain its project choice flexibility.