Roundabouts: A Round Peg in a Square Hole

Let’s begin with a scenario that could occur in any city or town across the country. A traditional, four-way intersection on the main road into a municipality’s downtown district has a higher-than-average crash rate, and residents have complained about high traffic speeds. Town leaders hire an engineer to study the problem and recommend a solution. After a comprehensive study, the engineer proposes the intersection be converted into a roundabout. Unfazed by the uncertain looks he receives for his suggestion, the engineer attends a meeting of the Town Council a week later to make his case for the conversion.

His report is met with skepticism from some council members and members of the public in attendance, who make their concerns known: roundabouts are confusing and unsafe, inconvenient for pedestrians, and expensive when compared to traditional intersections. Some even make half-joking references to Clark Griswold (of National Lampoon’s fame) driving hysterically around the roundabout near Big Ben in London.

The engineer explains the accuracy of growing data in support of roundabouts and how they moderate vehicle speeds, minimize conflicting vehicle and pedestrian movements, reduce the incidence and severity of crashes, and can aesthetically enhance an area, while increasing road system performance. Though acknowledging a roundabout is potentially

more expensive, the engineer details how the benefits outweigh the costs and explains that some funding programs do exist to cover portions of a roundabout’s construction cost.

In the end, the council backs the engineer’s expertise. A few residents remain concerned, but reserve further judgment. The town builds the roundabout, the intersection’s safety record improves, the speeding complaints dwindle, and community members grow accustomed to the change – their suspicions displaced by acceptance born out of familiarity and performance.

Why the resistance? The easy answer is because that is how it’s always been. We have grown comfortable with a traffic control system governed by automated signals – third-party referees who tell us when to stop, when to go, and when to yield. We motorists don’t even necessarily have to think that much, we just react; an increasingly dangerous proposition as our roads fill with ever more passive and distracted drivers. This tendency to passivity and distraction is especially hazardous at the most dangerous of traffic environments, the intersection.

The Federal Highway Administration (FHWA) estimates there are 300,000 signalized intersections in the United States. About one-third of all intersection fatalities occur at these locations, amounting to roughly 2,300 people killed each year. Furthermore,

ConceptsFishbeck, Thompson, Carr & Huber, Inc.

Spring 2013

onceptsC

Diverging 9 Merging 8 Crossing 16

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Diverging 4 Merging 4 Crossing 0

8

Standard Intersection

Potential Vehicle Conflict Points

Roundabout

Roundabout Safety Statistics• 38% reduction in all crash severities

• 75% reduction in injury crashes • 90% reduction in fatal crashes

www.ftch.com/concepts

There are only three rules for modern roundabouts:

1) Traffic moves counterclockwise.

2) Entering drivers yield to circulating traffic.

3) Stay in your lane.

This keeps traffic in the circle moving safely and contributes to a roundabout’s capacity and effectiveness.

about 700 people are killed annually in red-light running crashes. It is precisely this danger that has given rise to the FHWA’s effort to promote proven safety measures at intersections.

Roundabouts are one such measure. The FHWA says roundabouts have demonstrated substantial safety and operational benefits compared to most other intersection forms and controls, with especially significant reductions in fatal and injury crashes. In addition, they can be an effective tool for managing speed and creating a transition area that moves traffic from a high-speed to a low-speed environment.

Why are roundabouts safer than traditional intersections? The one-word answer is geometry. Roundabouts reduce the number of places one vehicle can strike another by a factor of four and do away with the left turn movement against oncoming traffic (arguably one of the most dangerous turning movement conflicts, resulting in severe T-bone crashes). Also, since modern roundabouts do not have traffic signals, they eliminate the scenario where a motorist speeds up to beat the red light. Drivers must slow down to enter a roundabout because traffic is directed around a center island, which also serves to reduce vehicle speeds. These lower vehicle speeds translate to overall intersection safety – the number and severity of crashes drops dramatically.

There is another factor that contributes to the safety of roundabouts: watchfulness. Drivers tend to be cautious when approaching and driving through a roundabout because it does not, initially, feel as comfortable as a signalized or stop-controlled intersection. This feeling can produce a heightened awareness of our surroundings; the benefits of which are obvious when compared to the passive distraction we all witness during our daily commutes. Roundabouts actively engage drivers, relying on their ability to assess changing traffic conditions and make reasonable decisions.

Significant design elements of a roundabout include a central circulation island and splitter islands on each approach. Safe pedestrian crossings can be integrated into the splitter islands. Many roundabouts are also designed with a truck apron, a raised section of pavement around the central island that acts as an extra lane for large vehicle turn paths. The back wheels of the oversized vehicle can ride up on the truck apron so the truck can easily complete the turn, while the raised portion of concrete discourages use by smaller vehicles. With a truck apron, roundabouts can

accommodate vehicles of all sizes, including emergency vehicles, buses, farm equipment, and tractor-trailers. The central island can also be designed and landscaped to allow emergency vehicles to traverse the center in extreme emergencies.

Public education is a critical, yet often overlooked, aspect to successful roundabout implementation. Many municipalities are setting aside resources for informational campaigns to teach their communities about roundabouts. These campaigns include mailings, brochures, online videos, and public forums.

There are few cure-alls when it comes to intersection safety, and roundabouts are not always the best solution. Some intersections do not have the right characteristics to consider a conversion. However, if designed properly and implemented effectively, roundabouts offer one of the safest, most efficient, and elegant alternatives to traditional intersection control. Points to ponder the next time you’re stuck at an empty intersection waiting for the signal to change.

Arrows show the direction of traffic flow

Approaching traffic slows and yields to traffic in the circle

Pedestrians cross at designated locations

Circulatory Roadway

Central Island

Yield Line

Inscribed Circle

Diameter

Landscape Buffer

Truck Apron

EntryExit

Accessible Pedestrian Crossing

Splitter Island

Roundabouts are easy to use

For more information regarding roundabouts, please contact Chris Sikkema, P.E. cjsikkema@ftch.com.