Ourston Roundabout Engineering

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.

Photo: New York State DOT

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

	Modern Roundabout	Nonconforming 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 proces

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.

Why Install Roundabouts?

Greater Safety: Roundabouts are the safest type of at-grade intersection. They create slower speeds, fewer conflict points for pedestrians and motorists, and reduced collision angles compared to STOP sign or traffic signal control. For more information, refer to Safety.

Reduced Delay: Since motorists do not always have to stop, delays and queues are usually less compared to STOP sign or traffic signal control especially at high traffic flows. For more information on their performance under heavy flows, refer to Avon and Vail.

Environmental Benefits: With reduced delay, roundabouts lessen noise, air pollution and fuel consumption. This is likely to become an increasingly important consideration in the future. Roundabouts also provide opportunities for traffic calming and speed transition, as well as landscaping and gateway treatments.

When societal costs associated with crash incidence are accounted for, roundabouts are often less expensive than other intersection control alternatives. A high-capacity roundabout can reduce the need to widen adjacent roads, a significant consideration with nearby underpasses or overpasses. Increased safety means lower emergency response, health care and other societal costs. Roundabouts do not specifically require periodic maintenance like traffic signals, and there is no need to be concerned with power outages.

Where Should Roundabouts be Considered?

Roundabouts are a feasible and practical alternative to other types of control where:

Traffic flows do not exceed about
~ 2,000 vehicles per hour for one-lane roundabouts; and,
~ 4,000 vehicles per hour for two-lane roundabouts; and,
~ 6,000 vehicles per hour for three-lane roundabouts; and,
~ 8,000 vehicles per hour for four-lane roundabouts.
Locations experience high rates of angle, rear-end or loss-of-control collisions.
Stop signs are creating unacceptable delays for side street motorists, but where a traffic signal is not warranted, or where a traffic signal would result in greater delays than a roundabout.
There is a high proportion of left turning traffic, or where the major traffic route is not straight through the intersection.
Intersections have unusual geometry or more than four legs.
It is important to emphasize the transition between urban and rural environments (i.e. gateways).

Roundabouts are not always practical or feasible where:

Land availability is limited.
Sight distance of the entry points is limited, such as on abrupt crest vertical curves on the intersection approaches.
Traffic signal progression is critical, as in some cases roundabouts can disrupt traffic platooning.