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time warner center-columbus circle-traffic circles-roundabout-nyc-untapped citiesColumbus Circle (a traffic circle, as shown by the walkways to access the center) viewed from inside the Time Warner Cable Building. (Image via Wikimedia)

This week we’ll celebrate Pi-Day on 3/14, so at HQ we’re thinking about circles. Did you know, the number of roundabouts and traffic circles around the world is in the thousands compared to only a couple hundred in the United States? The two things in the US refer to the same thing, generally speaking. Technically, the only thing that differentiates the two is the circle in the very middle. A traffic circle uses stop signs and/or signals to direct traffic, and allows people to traverse circular lanes and stop in the center. A roundabout on the other hand prohibits crossing to the center circle (as it is very dangerous) and only allows pedestrians to cross the streets that branch out from the circle. There are a few other technicalities left to talk about.

fuente-fountain-of-apollo-paseo del prado-madrid-spain-rondabout-traffic circle-untapped citiesThis roundabout in Madrid, Spain holds the spectacle of Apollo’s statue. But you’d have to be brave to safely traverse the circular lanes.

There are actually two kinds of traffic circles: those where the circling cars have the right-of-way and those where the entering cars do. The right-of-way is yielded by entering cars who must reduce their speed until a gap in the circle. Modern roundabouts take advantage of street engineering that utilize curves to create an environment that regulates speed, while traffic circles “emphasize¬†high-speed merging and weaving, made possible by larger diameters and tangential (straight) entrances” according to the New York State Dept. of Transportation.

In addition, the DoT explains one interesting difference between these two types of rotary:

In giving priority to entering vehicles, a traffic circle tends to lock up at higher volumes. The operation of a traffic circle is further compromised by the high speed environment in which large gaps are required for proper merging. These deficiencies have been essentially eliminated with the modern roundabout designs.

Stay tuned for more Cities 101 posts on Untapped Cities, getting you the information on how our cities work! You can look forward to  more from our Pi-Day festivities coming up. Get in touch with the author at @uptownvoice.

3 Comments

  1. Anthony Redington says:

    Glad that Patrick pointed out the 200 roundabouts in Wisconsin, a clearly among the leading U.S. states. He mentions the support of AARP whose transportation policies call for converting signals to roundabouts. As a member of the AARP’s Burlington (VT) Champlain Valley Action Team the AARP policy rests on sound data–while a quarter of all U.S. highway fatalities occur at intersections the percentage or seniors is over 50%. Seniors do suffer a decline in one identified key factor important in driving–judging speeds of approaching cars and safe “gaps” to move into traffic. That factor may well explain why the 90% reduction in serious injuries and fatalities at U.S. roundabouts is especially applicable to older drivers–lower speeds at roundabouts mean an easier judgement for seniors at the yield-at-entry roundies and the driver only deals with one traffic stream which approaches from the left. Also, a disproportionate, higher percentage of senior walking mode fatalities occur at non-roundabout intersections–again the walker benefits from a median refuge at a roundabout and dealing with a slower traffic stream from only one direction at a time. AARP and the Insurance Institute for Highway Safety are the two major national groups (Federal Highway Administration also) urging roundabout on all new busy highway intersections and a remedy retro-fit for the old ones. Tony Redington

  2. Wisconsin has over 200 roundabouts on the state highway system and about another 80 on the local system. I have the same information from AARP in Wisconsin favoring roundabouts over traffic signals.

    Our design criteria specifically addresses the design for large trucks and oversized-overweight (OSOW) vehicles, including OSOW lowboy loads, at roundabouts.

    We have worked with FHWA to attempt to minimize bridge width/cost between two diamond interchange ramp termini while meeting minimum requirements for low to medium bicycle and pedestrian needs.

    The WisDOT design guidance, photos, videos, animation flash video are all on the web site provided above.

  3. Tony Redington says:

    This article errors in estimates of U.S. modern roundabouts and traffic circles. U.S. Modern roundabout numbers total about 3,500-4,000 with about 500 being added each year. Traffic circle numbers are harder to quantify but certainly amount to several hundred–and are prime candidates for conversion to smaller, safer roundabout designs. My hometown of Keene, NH has four roundabouts going on five, New Hampshire and Vermont roundabouts now number almost 50, and several states, like Washington, roundabout numbers exceed 100. New York State’s DOT (Columbus Circle in NYC the first traffic “roundabout” in the world dates from 1905) “[modern] roundabouts first policy” has been in place since 2005 and other states and province transportation departments with similar policies include Virginia, Florida, North Carolina, Maryland. Alberta and British Columbia.

    It is true the U.S. trails leader France who built 1993-2003 roundabouts equivalent to 7,000 a year in the U.S. Most important, bicycle and walker mode depend on converting numbers of urban signals to roundabouts–and AARP supports signals to roundabout conversions because half of all senior highway fatalities occur at intersections versus less than 25% for other age groups. The single lane roundabout is really the “intersection safety belt.”

    Tony Redington
    Blog: TonyRVT.blogspot.com

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