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Experience the Roundabout
Jean-Ellen M. Willis, P.E., Engineering Manager - Transportation, and Tina Wawszkiewicz, P.E., Civil Engineer, City of Dublin, Ohio; Presenters, 2007 APWA Congress
The modern roundabout is always an option considered on intersection improvement studies for the City of Dublin, Ohio. We are experiencing improved safety and reduced delay at our four roundabout locations.
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Muirfield Drive and Brand Road roundabout installed on the heels of a fatal crash |
Our first roundabout solution came on the heels of a fatal crash at a stop-controlled intersection with a 120-foot-wide median. The intersection did not meet driver expectations, as east-west traffic had two stop-controlled intersections: one for the northbound through roadway and one for the southbound through roadway. There were also right turn bypass lanes to add to the confusion. The severity and frequency of the crash history and the large intersection footprint urged the City to investigate non-traditional solutions.
The modern roundabout was our best option, as the footprint was actually decreased and capacity was increased. We still had many skeptics, even some at the City! Concerns were raised about safety and driver confusion. Through research and a strong public involvement campaign, City staff and residents began to embrace the advantages of modern roundabouts and we have been able to implement several of them.
The public involvement campaign included the publication of two descriptive brochures, development of a website with a driving video and opportunity for comments, and multiple public meetings. The driving video was distributed to the high schools and driver education schools in the area so new drivers could learn appropriate driver behavior in the roundabout before bad habits were established. The website address was also posted on an informational sign on each roundabout approach for the first year of implementation. Although there was (and still is) some apprehension, the candid communication helped ease the transition.
Safety is arguably the greatest asset of the roundabout. Although our history is rather short with our first opening in 2004, we have seen substantial safety improvements. Crash rates before the roundabout at our first location were 2.7 crashes per million vehicles entering. After the roundabout, the rate is 1.0 crash per million vehicles entering. This represents a reduction of more than 60%. The types of crashes have changed too. The percentage of angle collisions and the severity of all collisions have both been reduced. The number of injury crashes has decreased by 75%. Travel speeds are lower and vehicles are never at right angles to each other. Speed differentials are decreased, reducing the frequency and severity of crashes.
At another intersection converted from stop sign to roundabout control in November 2005, we have seen great reduction in the severity of crashes, but lower improvement in the frequency of crashes. In this case, crash rates have changed by only 10%, but severity has improved by nearly 70%. At this location, the roundabout itself is two lanes, but upstream and downstream the through roadway is a single lane in each direction. Another geometric factor is that this intersection has three approaches rather than four, or in other words is a "Tee" intersection. We believe that the geometry may be playing a role in the relatively high crash rate. However, as with the first location, the number of angle-type collisions has dropped significantly. So, even with the small reduction in crash frequency, the safety has increased notably in the type and severity of crashes.
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Glick Road and Avery Road roundabout, the city's first single lane roundabout |
The other two intersections are single lane roundabouts. The Avery Road and Glick Road intersection shown in the photo was converted from two-way stop control. The remaining location was built at a newly formed intersection; the prior condition was a roadway link without access points.
Single lane roundabouts are more intuitive for both drivers and designers. The crash rates at these locations were relatively low prior to their conversion, and remain low with the roundabouts. The benefits we have seen at these locations are increased capacity, improved geometry, less noise, and lower travel speeds.
A challenge of implementing all roundabouts is designing intuitive signing and striping for an unfamiliar traffic control. We are still refining the signs and pavement markings. For instance, we experimented with large diagrammatic-type signs on the approaches. Residents did not like their appearance. They were too big and did not convey enough different information. We have been more successful with the introduction of street names on the lane assignment signs.
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Lane assignment with street name |
Street name signs within the intersection have also been rethought. We started with a more traditional sign with six-inch lettering. Taking the lead from Hilliard, Ohio, a neighboring community with two roundabouts, we have transitioned to 10-inch letting and integrated a right arrow into the sign.
We also tried using traditional lane assignment arrows on the approach pavement, including a block left turn arrow. This sent a confusing message to drivers; some even entered the circulatory roadway opposing traffic by turning left in front of the center island. The left turn arrows have since been removed from the pavement and single through arrows are used in their place. Left block arrows have been maintained in the circulatory roadway.
More lane guidance seems to work better in general. We have extended the lane line within the roundabout as a small dash to line up with the approach lane line. We have included spiral striping on the interior of the circulatory roadway with transverse lines to encourage drivers to stay to the outside of the roundabout. However, motorists seem comfortable ignoring this pattern and riding to the inside.
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A bicyclist rides comfortably through the Muirfield Drive and Brand Road roundabout. |
Pedestrians and bicyclists are treated uniquely with the roundabout approach. With the physical separation of opposing traffic with splitter islands, pedestrians only need to watch for traffic in one direction at a time. Bicyclists have the option of either riding through the roundabout as a vehicle or dismounting and crossing at the splitter islands as a pedestrian. At signalized intersections, pedestrians may have a false sense of security with a walk indication. By placing some responsibility on the pedestrian to watch for gaps and physically separating the conflict for the motorist between pedestrians and other vehicles, pedestrian safety is enhanced.
Other concerns we have heard about roundabouts are construction cost and land acquisition. We are finding construction costs are similar to a widening with a traffic signal installation. Although more land is needed for the immediate intersection area, by eliminating turn lanes and tapers, less land is needed for longer distances upstream and downstream of the intersection. Therefore, in most cases, less total right-of-way needs to be acquired.
Roundabouts have worked well for us in several different environments. We have had four successful installations and plan on many more.
The authors will give a presentation on this topic at the 2007 APWA Congress in San Antonio. Their session is entitled "Experience the Roundabout" and takes place on Monday, September 10, at 2:00 p.m. Jean-Ellen M. Willis can be reached at (614) 410-4633 or jwillis@dublin.oh.us; Tina Wawszkiewicz can be reached at (614) 410-4636 or twawszkiewicz@dublin.oh.us.
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