Roundabouts Vs. Traffic Circles and Rotaries - PDF by fyh14669


									                         Roundabouts Vs. Traffic Circles and Rotaries

Many folks believe that the circular intersections they have experienced in the eastern portion of
the United States are modern roundabouts. This is simply not true. For those travelers who have
only encountered old-style traffic circles, it is important to understand why modern roundabouts
move traffic safely and efficiently. There are major differences between the modern roundabout
and the old style traffic circle or rotary.

      There are three principals that explain the superiority of modern roundabouts over the old
traffic circles: yield rules, deflection, and flare.

            Roundabouts                                                 Traffic Circles

Yield-at-Entry Rule:                                          Entering traffic may interfere with
                                                              circulating traffic:
    •   Entering traffic yields to
        circulating traffic, which                                •   Circulating traffic can not
        always keeps moving.                                          clear when entering traffic
    •   Very efficient with heavy                                     fills circle.
        traffic.                                                  •   Heavy traffic causes
    •   No weaving distance is                                        gridlock.
        needed, so roundabouts are                                •   Circles must be large to
        small and fit in compact                                      provide long weaving
        spaces.                                                       distances.

Entering traffic is deflected slowly                          Inconsistent entry design may
around the central island:                                    allow traffic to enter at high
    •   Deflection controls speed
        without enforcement,                                      •   Serious accidents can result
        thereby reducing accidents.                                   on high speed streets.
    •   Deflection forms gaps in                                  •   Fast entries impede gap
        traffic so other vehicles can                                 acceptance and defeat the
        enter.                                                        yielding process.
    •   Entry flare adds lanes

Flare increases capacity at the                               Poor entry conditions may not
intersection, where capacity is                               benefit from flare:
needed most:
                                                                  •   Poor intersection capacity
    •   Flare promotes narrow                                         even with large traffic
        streets between                                               circles.
        roundabouts, saving cost                                  •   Higher capacity requires
        and neighborhood impacts.                                     wide streets between circles,
                                                                      wasting money and land

A visual explanation is sometimes superior to the verbal. The following page displays
two photos of old style traffic rotaries (in the United States) being converted to modern
roundabouts. The differences between the two traffic devices are obvious.
Kingston, NY – the outside larger traffic circle (rotary) is being replaced with the inner modern roundabout.

  Johnson City, NY – the outside larger traffic circle is being replaced with the smaller modern roundabout

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