Roundabout Designs That Work

Road Manager

Roundabout Designs That Work

Intersection locations, rural or urban areas, and other criteria determine the best criteria to use when designing roundabouts.

by Ruth W. Stidger, Editor-in-Chief

Roundabouts are especially useful to slow traffic and move it smoothly through city intersections. Usually these are single-roundabout designs.

Double-roundabout interchanges have begun appearing along freeways as a substitute for traditional diamond-design interchanges. The double roundabout eliminates traffic signals, some costs, and delays resulting from the signals.

Roundabout size is another key factor in successful design — and generally smaller is considered better.

Setting goals

Intersection designers pick modern roundabouts to help control delays and congestion. Improved safety is another goal, since about half of injury-related crashes and 20% of all fatality crashes happen at intersections.

In some cases software helps prove that roundabouts would do the job better than more conventional designs.

In a study, Guidelines for Preliminary Selection of Optimum Interchange Type for Specific Locations, N.J. Garber and M.D. Fontaine compared diamond interchanges with roundabouts at ramp terminals — but only in regard to traffic delays.

British software, which helps design roundabouts, predicts crashes and traffic flow, and is used to estimate delay and queuing for single-island roundabouts.

Dual-lane roundabout

Markings guide vehicle drivers

Islands at entries guide, too

Texas Transportation Institute software, Passer III, is used to determine diamond interchange cycle length, optimum signal timing, and possible delays.

The Federal Highway Administration’s Corridor Simulation software looks at alternative intersection designs.

According to a report by FHWA’s Joe Bared and University of Maryland’s Evangelos Kaiser, three specific cases for two-lane roundabouts and one case for a single-lane roundabout were included in the study.

Diamond, double-roundabout, and single-roundabout designs were examined.

Not included in the FHWA study, but worth noting, is the New York State Department of Transportation’s software, RODEL, an interactive program to design roundabouts.

Study findings

During off-peak and weekends, scenarios including roundabouts save up to 30 seconds of delay per vehicle, according to the FHWA study.

Until intersection vehicle throughput passed 5,000 per hour, both double- and single-roundabout intersection designs offered fewer delays than diamond designs.

Using Maryland State Highway Administration traffic trends, the FHWA study projects that in a year a double roundabout (compared to a diamond design) could save 35,000 vehicle hours for a single intersection with a throughput of 30,000 vehicles per day.

The study also shows increased safety with the use of roundabouts, partly due to the lower speeds at which vehicles enter the intersection.

Designs in place

European roundabouts have a long history, and the United States is beginning to move in the same direction.

Eric Teitelman, P.E., city engineer for Nashua, New Hampshire says that the city’s Broad Street roundabout offers many benefits over use of conventional intersections with traffic signals.

After the Division of Public works performed a due diligence analysis, the following comparisons were projected for the Broad Street location:

The main characteristic of a modern roundabout is the yield-at-entry rule, Teitelman says.

A circular center island to deflect vehicle paths and splitter islands to slow entry speed and deter illegal left turns are also key design points.

Today’s roundabouts, including the Broad Street location, are smaller than old rotary intersection designs. The Broad Street roundabout has an inscribed circular diameter of 120 feet, for example.

The smaller diameter slows traffic to 15 to 20 miles per hour — a big factor in the improved safety of the design.

Studies show increased vehicle safety of up to 75%.

A Norwegian survey of 181 intersections converted to roundabouts showed that the designs reduced pedestrian casualties by 89%.

In Colorado, the city of Loveland compared roundabouts to traffic signalized intersection performance.

One roundabout is located on a four-lane arterial with 20,000 ADT, says William Hange, P.E., city traffic engineer. Only 400 feet away is a major signalized intersection with 35,000 ADT.

After six years of use, there have been no injury crashes (including vehicles, pedestrians, and bicycles) on the roundabout.

The roundabout gave easier access to businesses previously affected by left-turn restrictions, and also provided better traffic flow and higher traffic capacity.

In Wisconsin, a roundabout at South Church Street in Watertown replaced a conventional intersection with heavy congestion and poor side-street business access, says Mark Johnson, P.E., transportation engineer with the Wisconsin Department of Transportation.

The roundabout improved business access and traffic flow, improved safety, and provided beautification opportunities.

In Colorado, a roundabout at Interstate 70 and the Main-Vail Diamond Interchange cost $3 million compared to $20 million for an improved conventional intersection — when it was first built nine years ago.

This roundabout reduced all crashes for the intersection by about 50%. Injury crashes were reduced by about 80%.

Traffic delays were also reduced and good business access was maintained.

In Pittsburgh, a roundabout project is underway at the intersection of Route 9 and York Road.

Roundabout construction cost will be about $110,000. Maintenance costs will be from $2,500 to $4,000 less per year than costs to maintain a signalized intersection.

The design will slow traffic entering the roundabout to about 20 miles per hour, says Don Adams with Sear Brown.

Roundabout Signing and Marking Are Critical

While agencies increased their use of roundabouts, the Manual for Uniform Traffic Control Devices’ directions for including proper roundabout signing and marking are minimal to say the least.

Single-lane roundabouts require less direction — yet still need signs and markings indicating entry points, travel paths, and exit points.

Multi-lane roundabouts create more complex signing and marking needs, says Christopher Kinzel, traffic engineer with HDR Incorporated, Kansas City, Missouri.

A two-lane roundabout, he says, requires:

Yield signs and direction signs at each entrance point.
Markings on entry and exit lanes.
Yield lines and signs at exit points.
Lane markings and exit path markings within the roundabout.

In multiple-lane roundabouts, generally left-turn exits may only be made from the inside or left lane, Kinzel says, while right-turn exits may only be made from the outside or right lane.

Solid-line markings can be used in heavy-traffic roundabouts to discourage lane changing, except at entrances and exits where dotted striping provides movement guidance, according to Kinzel. Not all designers agree with this concept.

Many roundabouts feature directional signs that combine street names. These may be variations of standard MUTCD destination signage.

Pavement arrows, adapted into a fishhook design, help prevent confusion for on-the-pavement markings or signage.

Redesign and signing

Mark Johnson at the Wisconsin DOT provided us with comments from Leif Ourston explaining the signing and marking used on Clearwater, Florida’s very successful roundabout redesign by Barry Crown. The original design used in Clearwater had many problems.

Crown’s crash reduction achievements show how well his redesign works. The roundabout had 522 fewer crashes per year, or a 99.6% reduction rate, compared to the original design.

Ourston, Ourston Roundabout Engineering, Santa Barbara, California, who has been in highway engineering for 44 years, calls Barry Crown the best roundabout engineer in the world. Crown redesigned hundreds of roundabouts in England and redesigned the badly performing roundabout in Clearwater.

Signing, striping, and marking make a big difference. Crown uses signs and stripes applicable to specific sites and sometimes realigns outer curbs a few feet for best results.

Spiral striping, rather than other less-effective signing such as a full-circle line and exit blisters (both part of the original, faulty design), is a key feature of the successfully redesigned Clearwater roundabout, Ourston says.

Ourston, whose Roundabout Design Guidelines is available at www.amazon.com, says that when roundabouts don’t work well, it is never the fault of the driver, the city, or the country. “It is always the design.”

Reprinted from Better Roads Magazine
July 2004