Roundabout vs. Traffic Circle

What is the Difference?

Roundabouts are not the same as traffic circles or rotaries. Traffic circles or rotaries have high-speed entries, weaving in the circle, low capacity, and many high-speed crashes. Sometimes motorists in the circle must yield to those entering. They are large and scary to drive – a “free for all” – and everyone hates them.

Roundabouts are the opposite. They require motorists to yield on entry. There is no weaving. Speeds are low, capacity is high, and crashes are few and minor. They are comfortable to drive, and if designed well, almost everyone likes them.

The photo below shows a roundabout being constructed within the central island of a large rotary in New York State. The roundabout is much smaller, yet will be safer and higher-capacity.

The specific design features that distinguish roundabouts from traffic circles and rotaries are Yield at Entry, Deflection, and (often) Flare.

 Modern RoundaboutNonconforming Traffic Circle


Yield at Entry

Entering traffic yields to circulating traffic.
  • Circulating traffic always keeps moving.
  • Works well with very heavy traffic.
  • No weaving distance necessary. Roundabouts are compact.
Entering traffic cuts off circulating traffic.
  • Circulating traffic comes to a dead stop when the circle fills with entering traffic.
  • Breaks down with heavy traffic.
  • Long weaving distances for merging entries cause circles to be large.


Deflection

Entering traffic aims at the center of the central island and is deflected slowly around it.

  • Slows traffic on fast roads, reducing accidents.
  • Deflection promotes the yielding process.
Entering traffic aims to the right of the central island and proceeds straight ahead at speed.
  • Causes serious accidents if used on fast roads.
  • Fast entries defeat the yielding process


Flare

Upstream roadway often flares at entry, adding lanes.
  • Provides high capacity in a compact space.
  • Permits two-lane roads between roundabouts, saving pavement, land, and bridge area.
Lanes are not added at entry.
  • Provides low capacity even if circle is large.
  • For high capacity, often requires multilane roads between circles, wasting pavement, land, and bridge area.

 

Traffic circles constructed for traffic calming purposes, below, tend to be small and of low capacity. Large vehicles are often not accommodated, or must turn left in advance of the circle in opposition to other traffic. Many do not have splitter islands, which direct motorists and provide refuge areas for pedestrians.

For more information, the Insurance Institute for Highway Safety has a good Q&A on Roundabouts.