THE EMERGING CHOICE FOR INTERSECTION DESIGN

The modern roundabout differs from those built early in this century. Modern roundabouts operate on the ‘yield to traffic in the circle’ rule, have entry deflection to slow vehicles, and can have flared entry points to increase capacity. In addition, they are relatively small and low speed. Typically they are from 45 feet to 200 feet in diameter and operate between 15 mph and 25 mph. Any circle that is greater than 200 feet diameter with operating speeds greater than 25 mph is NOT a modern roundabout. These differences allow the modern roundabout to operate as the "safest, most efficient and attractive form of traffic control in the world" (Michael Wallwork, 1999).

The first Modern Roundabouts were built in the USA in 1991 and 1992. Any circular intersection built in the USA before 1991 is NOT a Modern Roundabout.

The City of Manhattan built the first modern roundabout in Kansas in 1997. A team of Kansas State University researchers has studied the safety and operation of this modern roundabout. The results of this research show:

• This modern roundabout operates as well or better than comparable intersections with other types of traffic control;

• This modern roundabout operates better than the 4-way STOP alternative which was considered prior to roundabout construction; and

• This modern roundabout has operated for over three years with only one minor traffic crash. This compares to an average of over three (with at least one injury crash) per year prior to construction.

The final report for this research has been compiled. The full report is available from the Civil Engineering Department at Kansas State University or can be viewed on the Mack Blackwell website - http://www.mackblackwell.org/ . (Click on "Research" then "Final Reports". Next under Traffic Managaement, click on "Modeling Traffic Flows and Conflicts at Roundabouts (1099)".)

The Kansas State University research team has begun examination of a number of other roundabout sites either under construction or planned throughout the state of Kansas, Maryland and Nevada. Watch this site for information and updates on these ongoing research efforts.

After three years of research for the Kansas DOT, (KDOT: www.ink.org/public/kdot/index.html) Mack Blackwell, National Transportation Research Center (MBTC: www.mackblackwell.org) and the Insurance Institute for Highway Safety (IIHS: www.hwysafety.org), the overall conclusion is that the Modern Roundabout is the safest and most efficient form of intersection traffic control available today.

Safety: In a recent IIHS study of 24 intersections in the USA where stop control and traffic signals were replaced with Modern Roundabouts, there was a:

• 39% overall crash reduction

• 76% injury crash reduction

• 90% fatal crash reduction

Efficiency: Our studies show significant reductions in vehicle delay, queue length and stopping. (Actual numbers will be made available in future sections)

SOME CLARIFICATIONS OF MISUNDERSTANDINGS REGARDING MANHATTAN ROUNDABOUTS

Anyone has a right to dislike roundabouts and "The Manhattan Mercury" (the local newspaper) has a right to put anything in print. However, it would be nice to separate some facts regarding roundabouts from much misinformation that has appeared in print over the past year or two.

Fact One: A modern roundabout is a traffic control device with the primary function of controlling traffic movements through an intersection.

Fact Two: There are only two modern roundabouts in the Manhattan area. These are located at Gary/Candlewood and Kimball/Grand Mere.

Fact Three: All those other "round things" are traffic circles, with the primary function of calming (slowing) traffic along a street. (Think speed bump, i.e., a speed bump every 400 feet would serve the same function.)

Fact Four: Modern roundabouts are not those big, old, high-speed traffic circles built years ago in Paris, London, Washington D.C., and other eastern cities. These circles were designed for high-speed entry and multilane weaving - a dangerous combination. The potential for serious crashes is high and subsequently, most traffic circles generally have high crash rates.

Fact Five: The modern roundabout is the safest form of intersection control available. This fact has been verified worldwide and in the USA by several recent studies of intersection safety. In a recent Federal Highway Administration publication ("Roundabouts: An Informational Guide." USDOT/FHWA Publication No. FHWA-RD-00-067, Washington, D.C., June, 2000) it is reported that: "experience in the United States shows a reduction in crashes after bulding a roundabout of about 37 percent for all crashes and 51 percent for injury crashes." If only small to moderate single lane roundabouts are considered, the reductions are 51 percent for all crashes. Additionally, reductions are 73 percent for injury crashes. Mean reductions in crashes after converting to a modern roundabout from other traffic control devices have been similar in several other countries: Australia 41-61% for all crashes and 45-87% for injury crashes; France 57 to 78% for injury crashes; Germany 36% all crashes; Netherlands 47% all crashes; United Kingdom 25-39% injury crashes (p. 112, Exhibit 5-9). In probably the most comprehensive US study to date, the Insurance Institute for Highway Safety recently completed an in-depth study of 24 intersections which were converted from stop control and signals to modern roundabouts during the past decade. These 24 intersections were a mix of urban, suburban and rural environments. Overall, the study found reductions of 39% for all crash severities combined, 76% for all injury crashes and an approximate 90% reduction in fatal and incapacity injury crashes ("Crash Reduction Following Insallation of Roundabouts in the United States." Insurance Institute for Highway Safety, Arlington, VA, March, 2000).

In regard to pedestrian safety, US data is too limited to draw meaningful conclusions; however, several other countries report significant reductions in both numbers and severity of pedestrian crashes at modern roundabouts. For example, a Dutch study of 181 intersections converted to roundabouts reported a 73% reduction in all pedestrian crashes and an 89% reduction in pedestrian injury crashes (FHWA-RD-00-067).

Fact Six: The modern roundabout is the most efficient traffic control device that exists today, based on overall vehicle delay and queue length (number of vehicles backed up). This fact also has been proven by several studies in the US, (including several at KSU) as well as worldwide.

Gene Russell

For more information on Roundabouts and how do they differ from Traffic Circles, go to the Environmental and Transportation Engineers and Environmental Scientists site

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