Say it three times: “Roundabouts are NOT traffic circles.” The first roundabouts in the United States were built in 1990 in Summarily, Nevada. Anything built before that time is a traffic circle and not a roundabout. New Jersey has lots of traffic circles, but they do not have ANY roundabouts. The two are distinctly different. The pervasive misconception and insistence that roundabouts are the same as traffic circles is one that must be corrected if we are to move out of the dark ages of road design and into an era of modern mobility.

"Roundabout in Western Maryland. Not your typical Jersey Circle, is it?

What is the difference?

Traffic circles simply do not work and their failure is based on the fundamental design parameters that call for high-speed entry, high-speed merging and high-speed exiting. The goal of a traffic circle is to keep cars moving on the primary road at as high a speed as possible, which is accomplished by the use of large center islands and tangential primary entry and exit points. This results in design speeds of 25 - 40 MPH and actual speeds that are often much faster. Of primary concern is that this forces entering vehicles to merge with vehicles already in the circle at high speeds which virtually guarantees that any accident that occurs will be significant.

Circles seem to function on the singular rule that “there are no rules.” In more scientific terms, circles operate on the principle of merging. Approximately 240 feet is required for safe merging at 40 MPH and very few circles actually provide this distance. This is a very fundamental design flaw and is the primary reason that circles should have never been constructed. They were built to fail.

Roundabouts differ from circles in several critical aspects. First, the central islands are much smaller, generally 15 to 120 feet in diameter as compared to a minimum of 300 feet for a circle which results in lower internal speeds. Second, and most importantly, vehicles never enter the roundabout on a direct tangent but are always deflected. This is generally accomplished by the positioning of a triangular island at the entry and exit points that forces the traffic to turn into or out of the roundabout. The geometry of the design results in much lower speeds (generally 10 - 25 MPH) and allows the roundabout to function on the principle of “gapping” rather than merging. Those entering the roundabout determine when the gap is large enough (and at 10 - 25 MPH it is much smaller than it is at 40 MPH), and then proceed. While merging is a high-speed endeavor, gapping involves a low-speed, controlled movment into traffic when there is ample space between moving cars. A third critical distinction is that all vehicles entering the roundabout must yield to those who are already circulating. There is no confusion as to the rules that apply although an initial learning curve is often required.

If a circular traffic structure does not contain these three elements it is NOT a roundabout:

Although not an absolute requirement to be considered a roundabout, there are several additional elements that are commonly found in their usage. For example, entry roadways are often flared before the yield line to increase capacity. Pedestrian crossings, if provided, are located before the flare. Additionally, the center islands are often constructed with an apron to provide extra turning room for large trucks and emergency vehicles.

Safety

Of all intersection types, cross intersections are the most dangerous. This applies to both those that are signalized and those that are controlled by signage. The accidents that occur at perpendicular intersections tend to be more severe as people either speed up in an attempt to “make the light” or “run the stop sign,” either of which creates the potential for devastating collisions. To illustrate the severity of the problem, the Federal Highway Administrations reports that in 1999 there were 85,000 crashes that resulted from drivers running red lights and that those crashes killed over 1,000 people and injured 81,000 more. Traffic circles also present safety issues as their design encourages high speed merging and the confusion that they generate often results in driver error.

Roundabouts, in contrast, have proven to be remarkably safe. Studies by both the Insurance Institute for Highway Safety and the Federal Highway Administration have found that when roundabouts are used to replace traditional intersections that the accident rates are lowered significantly. The Insurance Institute reports a 39% decline in accidents, a 79% decrease in injury producing collisions and a 90% decrease in fatal or incapacitating crashes. The results of these studies mirror the experience of numerous European countries that utilize roundabouts in place of traditional intersection design.

The safety of roundabouts is a direct result of their design. When compared to a sign or signal controlled 4-way intersection, a roundabout has 8 conflict points compared to 32 for the traditional design. The result is almost always fewer collisions and those that do occur are usually low-speed rear-end or merge collisions rather than the high-speed angle crashes that are common at perpendicular intersections.

Diagrams
(Please see refer to the following links for more complete details on the safety of roundabouts.)
http://www.hwysafety.org/srpdfs/sr3505.pdf and http://www.tfhrc.gov/safety/00-0675.pdf

Mobility

Most Americans are familiar with the story of the tortoise and the hare and many have used the story to try to teach their children valuable life lessons of “slow and steady wins the race.” Unfortunately, many people fail to see that the principles in the fable also apply to the roads that they so frequently drive.

Traffic lights are the single biggest impediment to free flowing traffic. They force people to stop. How many times have you seen someone zoom away from a traffic light at an unreasonable speed only to catch up to them as they wait for the next light? How many times have you waited for a light even though there are no cars utilizing the opposing green light? You can rush all you want but the traffic lights are going to slow you down. Even synchronized lights do not do the trick. You are just as likely to have to stop for every light as you are to get a clear run.

It is really very simple: If you want to increase travel times, put in a traffic light. If you want to reduce capacity on a road, put in a traffic light. If you want more congestion, put in a traffic light.

One of the reasons that roundabouts are not well accepted by the public is that they are classified as a “traffic calming” device. People respond to them by saying; “Who needs traffic calming- it is already backed up.” This argument makes no sense because by eliminating the lights at an intersection, roundabouts really “speed” up traffic. No one has to wait for a light, everyone keeps moving, albeit slowly, at speeds around 10 - 25 MPH. Thus, we get back to the tortoise and hare analogy and ask, “Which would you rather do: wait for the traffic light (assuming you make it through the first one) and speed toward the next light or would you rather keep moving through the intersection?” Hopefully, you choose the latter because it has been proven over and over again that it is better to keep moving. Congestion is reduced as are total drive times.

Studies in Maryland, Kansas and Nevada have shown sizable average reduction in both the number of stopped vehicles (-14% to -37%) and in delays (-13% to - 23%) when traditional intersections have been replaced with roundabouts. (Source: Insurance Institute for Highway Safety) Even more dramatically, the Federal Highway Administration reports that the construction of a large roundabout in Towson, Maryland (built to handle nearly 50,000 vehicles a day) “substantially improved the Level of Service (LOS) and decreased the traffic delay at the location. The morning LOS went from B to A and the evening LOS improved from E to B. Total vehicle delay was lowered by 70% and 58% in the morning and evening peak hours, respectively.”

The capacity of a roundabout is typically much higher than a controlled intersection because traffic can enter from each approach simultaneously. In a controlled intersection, only a portion of the center area is being utilized at any given point, because of left-turn signals, the delay between signals, and the alternation of traffic from opposing sides. One can visualize this by thinking of an intersection from an aerial view, and noting that this use of road space is extremely inefficient. With roundabouts, the road is being maximized at all times. One-lane roundabouts generally have a capacity of 2500 - 2800 vehicles per hour and are especially useful for intersections that experience high left turn rates because no additional delay is introduced as left-turning and through traffic enter the roundabout in the same fashion.

Public Acceptance of Roundabouts

At first blush, the concept of roundabouts in the United States generates an immediate and negative reaction. However, once they are in use and people become familiar with them, the sentiment does indeed change. In a recent report by the Insurance Institute for Highway Safety, drivers in Kansas, Maryland and Nevada were polled before the construction of roundabouts in their areas and 41% of those surveyed strongly opposed their construction. Following the completion of the roundabouts that number dropped to only 15%. Likewise, while only 31% favored roundabouts prior to their construction, that number grew to 63% once they were in use for several months.

Failures and Fairness

Just because it is called a roundabout does not mean that it is well designed. A poorly designed roundabout will perform poorly as will a poorly designed intersection of any type. Perhaps the most often cited example of an improperly designed roundabout is the largest roundabout in the country located in Clearwater, Florida. When it initially opened, there was a massive increase in the number of accidents and, seemingly more importantly to many, the fountain in the middle of the roundabout sprayed water on passing cars.

The problem with the Clearwater roundabout was threefold: 1.) The entry points were improperly angled allowing cars to enter at too high of a speed as well as allowing some drivers to go the wrong way; 2.) The signage was very poorly configured; and 3.) The fountain limited the driver’s visibility - a far more serious problem than merely spraying newly washed cars.

Although it has been claimed that the roundabout was such a fiasco that it was removed, that is not the case. The fountain was eliminated, the entry points correctly constructed and the signage was improved. The result? An instant reduction in the accident rate from 24 per month to zero and a functional intersection capable of handling up to 58,000 vehicles a day without congestion. (Please see http://www.alaskaroundabouts.com/wsj-roundaboutResponse.htm for additional information on the corrective action taken to remediate the Clearwater roundabout.)

The point of this example is that it is not appropriate to condemn an idea because it was poorly implemented in one instance. To do so simply isn’t very bright. Likewise, to accept roundabouts carte blanche would be equally inane.

For example, the roundabout in Towson, Maryland that was discussed earlier has done an excellent job of relieving traffic pressure, but it has been the exception to the rule when it comes to accidents. There has been a fourfold increase in property loss accidents (damage to vehicles) since it opened (2.6/yr vs 10.0/yr). Fortunately, that has been offset by a two-thirds reduction in accidents that result in physical injury. (Source: Federal Highway Administration) Thus, while the intersection is much safer for people, one could make the argument that it is not such a great place to be if you are a car.

We would suggest that rather than condemn roundabouts because the one in Towson has seen an increase in minor accidents, it would be more appropriate to evaluate the totality of the situation. Is a minor accident every five weeks a reasonable occurrence given the traffic volumes? If it is higher than average, is the trade-off in terms of reduced injury accidents one that is worth making? Should the increased mobility and revitalization of the economic base near the roundabout be factored into the equation when determining if the increase in minor accidents is “acceptable?” The Towson roundabout is a relatively rare two-lane design. Are two-lane roundabouts inherently more prone to accidents than single lane designs and if so, are there design changes that can be made to reduce their frequency?

Are roundabouts for radicals only?

Although roundabouts are widely accepted in Europe (the British, French and Swedes are close to banning traditional cross intersections entirely,) their acceptance in the US has been slow and somewhat torturous. This is changing rapidly, and roundabouts are becoming widely accepted in the transportation and planning communities. One can now find roundabouts popping up throughout the country where they are being used for everything from small traffic calming devices in residential areas to large gateway intersections handling over 50,000 vehicles per day.
Below are links that will take you to the Federal Highway Administration and the Insurance Institute for Highway Safety. These sites have been chosen both because they provide a wealth of valuable information on roundabouts and because we doubt that anyone will accuse them of being radical entities. We are trying to be fair and reasonable and hope that those who are reading this will be as well.

Roundabouts: An Informational Guide - Federal Highway Administration - This first link provides an overview while the second is an extremely comprehensive guide to roundabouts from the basics to engineering design standards.

http://www.tfhrc.gov/pubrds/fall95/p95a41.htm

http://www.tfhrc.gov/safety/00068.htm

Insurance Institute for Highway Safety Documents - These documents provide both operational explanations as well as detailed information on roundabout safety.

http://www.hwysafety.org/srpdfs/sr3607.pdf

http://www.hwysafety.org/srpdfs/sr3505.pdf