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Roundabouts vs. Rotaries   

A Roundawhozit?

Don't we call those things Traffic Circles? And don't Rotaries meet in pool halls and play bingo? A friend asked me the other day what the difference is between a traffic circle and a roundabout. I said, "Ummm, well, you see..." I'm supposed to be able to answer questions like that. I am a traffic engineer after all.

Here are the results of some research:

  1. "Roundabout" refers to modern circulatory intersections designed for increased capacity and safety over standard stop-controlled intersections. British transportation engineers were responsible for the modernization of these designs in the ‘60’s

  2. "Rotary" usually refers to older, high-speed circulatory intersections, with large diameters up to and past 100 meters. They may operate with the encapsulated traffic yielding to the entering traffic.
  3. "Traffic Circle" usually refers to small neighborhood circles and/or traffic calming devices operating with or without stop or yield control.

I've mentioned the type of yield or stop control several times. This is very important to the operation of the intersection. If you allow entering vehicles free access to the circulatory intersection, i.e. make the circulating traffic "yield to the right" (directions are for U.S. Traffic) then, at peak times, entering traffic can easily overwhelm the capacity of the intersection leading to congestion, collisions, and delay. Modern roundabouts do not permit traffic to enter with impunity and instead must yield to traffic already within the intersection.

Safety:

Roundabouts offer increased safety at intersections by reducing the severity of the collisions that occur. The frequency of collisions may go up, but the number of injuries and fatalities go way down. Roundabouts accomplish this by eliminating two of the most hazardous collision modes: right-angle and left-turn head-on collisions. The design of the roundabout forces drivers to slow, reducing the absolute speeds at which collisions occur. Finally, the drivers inside the roundabout tend to be moving at similar speeds, reducing the relative speeds in collisions.

Pedestrians experience reduced hazards in roundabouts with respect to a traditional intersection. Pedestrians do not have as many places to look in order to detect oncoming traffic while crossing the street. The splitting islands provide pedestrian refuges during a crossing maneuver. Speeds, as mentioned, tend to be lower, and the crossings tend to be shorter.

Bicyclists face higher injury-accident rates at roundabouts than at more traditional intersections. Typical crash modes include sideswipes from entering or exiting vehicles, as the bicyclist circulates in the roundabout.

Some Basic Assumptions that do not necessarily hold under complex circumstances:

  1. Roundabouts will always provide greater capacity and lower delay than an all-way traffic controlled intersection
  2. Roundabouts will not provide lower delays at intersections where the minor street is not (and is not projected to) have operational problems under two way stop control
  3. Roundabouts can be assumed to be operating within capacity if the intersection doesn't exceed the peak hour signal warrant
    • Roundabouts operating within capacity will provide lower delays than the same intersection under signal control
    (the above taken from section 3.4.4 of http://www.tfhrc.gov/safety/00-0673.pdf)

    Some Notes:

    • Typical roundabout volumes of up to 20,000 vehicles per day on non-compact roundabouts with 4 legs and one entering lane in each direction
    • Bicycles and Pedestrians require extra attention due to the nature of the intersection. Traffic never stops, allowing few gaps for slow moving pedestrians or bikes to cross the travel lanes.
    • Roundabout intersections inside signal systems may operate better *as a system* under signalized control for platooning purposes. A platoon of vehicles is a segregated group of vehicles advancing through a traffic signal system. A non-signalized roundabout would not provide any degree of segregation.

Items from the Federal Manual on Uniform Traffic Control Devices (MUTCD) 1A.13

11. Circular Intersection—an intersection that has an island, generally circular in design, located in the center of the intersection where traffic passes to the right of the island. Circular intersections include roundabouts, rotaries, and traffic circles.
68. Roundabout Intersection—a circular intersection with yield control of all entering traffic, channelized approaches, and appropriate geometric curvature, such that travel speeds on the circulatory roadway are typically less than 50 km/h (30 mph).

http://mutcd.fhwa.dot.gov/HTM/2003/part3/part3b2.htm#section3b24 Roundabout Markings
Source: Federal Highway Administration Manual on Uniform Traffic Control Devices 2003

http://mutcd.fhwa.dot.gov/HTM/2003/part2/fig2c-08_longdesc.htm