Chapter 5 - Roundabouts

Introduction

A roundabout is a circular intersection between streets in which traffic flows in one direction around a circular roadway from the point of entrance to the point of exit. It usually requires more land space than a regular cross intersection, but has several advantages. One of these advantages is slow, but steady vehicle speeds and traffic flow. These slow speeds make roundabouts effective traffic calming measures. Typically people associate traffic calming measures with a decrease in vehicular capacity. However, roundabouts not only slow vehicle speeds, but also improve capacity for many large intersections.

Roundabouts are defined by a yield-at-entry rule for entering traffic, as opposed to the smaller neighborhood traffic circle, which gives entering vehicles the right of way while vehicles in the circular roadway must yield. Yield-at-entry is the most important operational element of a modern roundabout. Other important features of a modern roundabout include:

• Deflection of the vehicle path to slow down traffic before entering the roundabout.
• Entry flare of approaching lanes.
• Splitter island at all approaches to control entry speed and deter left turns.
• A truck apron (in small roundabouts where large trucks need extra space to turn).
• Good sight distance, lighting, and signing.
• No crosswalks across the circulatory roadway.
• Yield lines downstream of the pedestrian crossings.
• No parking in the roundabout.

For large multi-lane roundabouts, entry flare is often used. This is the widening of the road approaching the roundabout. It provides more lanes close to the roundabout entry so that, as vehicles slow down when approaching the roundabout, they may also spread out into more lanes before entering the roundabout. This prevents long queues from forming and provides a high capacity at the roundabout. A good flare of approaching lanes will give a large roundabout as much entry capacity as all the narrow linking roads that join the roundabout.

Roundabouts vs. Cross Intersections

Although roundabouts require more space than 4-leg cross intersections, they have many advantages over cross intersections. These include:

• Improved safety.
• Increased capacity compared to a three or more phase signalized intersection.
• Lower cost.
• Environmental and aesthetic improvements.

Improved Safety

When built at appropriate sites, the modern roundabout is considered the safest type of intersection. “Roundabouts are normally the safest form of at-grade junction over a wide range of entry flows and approach speeds.”¹ Reports from the U.K. and France indicate that accident risks at roundabouts range from 10 to 30% of those at conventional cross intersections. Reduced speeds at roundabouts have been shown to be the primary cause of improved safety. Another factor is the reduced number of conflict points as compared to conventional intersections.²

There are essentially three types of road intersections: cross, T, and Y intersections. A cross intersection has four legs, usually at a 90-degree angle with one another. T intersections have three legs, also usually at a 90-degree angle with one another. Y intersections have three legs, usually with one leg at an angle less than 90 degrees, much like the letter “Y”. These intersections are regulated by either traffic signals, stop signs, or yield signs. Studies have clearly indicated that cross intersections are more dangerous than T intersections and Y intersections, and that traffic signal regulation is more dangerous than stop sign regulation.¹ This is the expected result, as 4-leg intersections are more complex than 3-leg cross intersections, and signalized intersections generally have higher speeds and more traffic than stop-sign intersections. Studies have also clearly shown that T intersections are safer than Y intersections. This result is logical because in Y intersections, a driver must look backwards over his shoulder or use side mirrors to merge with traffic, sometimes resulting in blind spots. T intersections, on the other hand, only require the driver to look 90 degrees to the left and right, which increase the chance that the driver will see all oncoming vehicles.

These results about the safety of the three types of road intersections are important in the comparison of roundabouts with cross intersections. First of all, roundabouts never include cross intersections. All the legs of a roundabout intersect with the circulating roadway as T or Y intersections. This makes roundabout intersections safer than cross intersections. Also, roundabout entrances are safer than regular Y intersections. Since deflection is a design objective of a good roundabout, designers aim to make the entering legs more of a T intersection with the circulating roadway than a Y intersection, so drivers will need to slow down before entering the roundabout. This deflection is accomplished by aiming the entering lanes toward the central island of the roundabout, and ensures that drivers will enter the roundabout at safe speeds. This also enhances the safety of a roundabout.

Low Frequency of Roundabout Accidents: Based on the above analysis, one would expect that roundabouts experience fewer accidents than cross intersections. This is exactly the case. In the Netherlands nine intersections regulated by traffic signals were converted to roundabouts. There was a 27% reduction in accidents and a 33% reduction in casualties. In Norway, accident records from 1985 to 1988 were examined for 59 roundabouts and 124 signalized intersections.¹ The results are shown below in Table 5.1.

Table 5.1: Number of Reported Accidents per Hundred Million Vehicles in Norway Study

The Norway study is one of several studies that proved roundabouts are safer than signalized intersections. Based on various studies like the Norway study and other sources of information, the following is a reasonable rule of thumb: at a site where a roundabout and a signalized intersection are both possible alternatives, one can expect twice as many accidents with a signalized intersection than with a roundabout.¹

Low Severity of Roundabout Accidents: In 1990 Britain determined that 0.43% of crashes at roundabouts involved fatalities, whereas 1.3% of crashes at all other intersections involved fatalities. The U.K. also reported that the average cost of damage per crash at roundabouts was about half that at other intersections.³ These results show that generally accidents are less severe at roundabouts than at other intersections. This is the expected conclusion, since roundabout traffic is generally flowing in the same direction and travelling at comparable speeds.

Increased Capacity

Roundabouts not only reduce accidents, but increase capacity. Compact single-lane roundabouts, sometimes called mini-roundabouts if the diameter is approximately 30 meters, operate quite well at average daily traffic levels below 15,000 vehicles per day (vpd) and can handle up to 25,000 vpd. Traffic volumes higher than this can be accommodated by multi-lane roundabouts. Comparing a roundabout to a signalized intersection, if the signal has four or more phases, roundabouts generally result in a higher intersection capacity.² During off-peak hours, roundabouts result in little or no delay, whereas a signal will always result in a delay. At a normal signalized intersection, there is a lot of “dead time” in which no vehicles are crossing the intersection, while other vehicles are waiting to cross. At a roundabout, a driver only needs to yield to the traffic in the roundabout and then make the judgment when it is safe to enter. By eliminating the computer regulation of a traffic signal, roundabouts completely eliminate this inefficiency of dead time and therefore increase capacity.

Lowered Costs

Roundabouts in general are less expensive over a long period of time than a computer operated traffic signal. Construction costs of roundabouts vary widely, from $10,000 to $500,000. Roundabout interchanges (discussed below) cost $2.8 million to $6.4 million to construct. Traffic signals require computer operation and much more maintenance, which is costly. Depending on the complexity of the intersection, traffic signals can be very expensive both to install and operate.

Roundabouts also prevent costly road widening for capacity improvements. Many times at freeway off-ramps, limited space is available for queuing. Road widening is sometimes believed to be the only solution to increase capacity and provide queue storage space. Roundabouts as freeway-to-street interchanges at these locations can help reduce queuing on freeway off-ramps that is caused by traffic signals. This use of roundabouts is called “wide-node / narrow-road” highway engineering.⁴

The idea is that by increasing capacity at nodes by constructing large roundabouts (“wide nodes”), narrow link roads can join the nodes, avoiding road widening for an overall cost saving. Caltrans explains this concept under “Reduction of Queue Storage Requirements,” in Design Information Bulletin 80:⁴ Roundabouts can produce operational improvements in locations where the space available for queuing is limited. Roads are often widened to create storage for vehicles waiting at red lights, but the reduced delays and continuous flows at roundabouts allow the use of fewer lanes between intersections. Possible applications may be found at existing diamond interchanges, where high left turn volumes can cause signals to fail. By constructing a pair of roundabouts at the ramp intersections, capacity improvements to the interchange can be accomplished without the costly requirement of widening the structure to carry additional lanes over or under the freeway.

Signalized interchanges require wide bridges for storage and turning lanes. Therefore roundabout interchanges are usually much less expensive than signalized interchanges because wide bridges are not necessary. To retrofit existing interchange with two roundabouts costs about $2 to $6 million. To install signals and widen a freeway bridge costs about $10 to $15 million. It is clear that in a case when road widening is expensive, roundabouts provide a better solution.

Example: Vail, Colorado is an example of creating high capacity at a low cost by widening the nodes, not the link joining them. The town built North America’s first modern roundabout interchange at I-70 / Vail Road in 1995, replacing two adjacent freeway off-ramp intersections, which were regulated by stop signs, with two roundabouts. Long queues of traffic used to extend back onto the freeway. People used to wait in the traffic as long as half an hour on peak ski days just to enter or leave Vail. Now, even under the heaviest traffic, queues rarely exceed 10 vehicles. The waiting times have been eliminated, thereby reducing air pollution and fuel waste.⁵ Another solution Vail considered was to widen the underpass of the freeway. The freeway bridge would need to be lengthened to accommodate this, which was estimated to cost $5 million. The total cost of constructing the two roundabouts was $2.8 million. The roundabouts also save the town $85,000 per year on traffic direction officers, who are no longer needed. Residents overwhelmingly approve of the roundabouts and expressed appreciation of the aesthetic improvement the new design brings to the entrance of their world-class resort.⁵

Environmental and Aesthetic Improvements

There are many other advantages of roundabouts over cross intersections. Just as the Vail residents appreciated the aesthetic improvements the roundabouts brought to their town, roundabouts in general are a visual improvement over signalized intersections. The center of the inner circle can be nicely landscaped to become the center of attention. Another advantage of roundabouts is that by allowing vehicles not to stop before entering roundabouts, noise and emissions are reduced from the stop-and-go driving associated with signalized and stop-sign regulated intersections. Roundabouts also have environmental benefits. They generally require 20 to 30 percent less impermeable surface area than cross intersections and consequently reduce adverse environmental impacts associated with runoff.²

Bicyclists and Pedestrians

Some people may consider roundabouts not to accommodate bicyclists and pedestrians very well. However, bicyclists and pedestrians can be sufficiently accommodated in roundabouts. Bicyclists are expected to ride around the outside of the roundabout to their point of exit, while exiting and entering vehicles yield to them. According to the Florida Roundabout Guide, “no special markings or lanes are generally needed in the roundabout to accommodate bicyclists.”⁶ However separate lanes have been used in some high volume roundabouts.

Large roundabout designs can result in greater walking distances and thus inconvenience pedestrians. Well-designed splitter islands, sufficiently large to hold pedestrians, will enable pedestrians to move safely around the intersection, crossing only one direction of traffic at a time. Pedestrians must identify and accept gaps in traffic and to cross when it is safe to do so. This is less convenient for pedestrians compared to a signalized intersection where vehicular traffic yields to them, rather than them yielding to traffic.

Roundabout Design Technology

Because Britain started the roundabout era and has more experience with roundabouts than other countries, they have produced the best design-aiding computer software. Three computer programs which designers use include RODEL, ARCADY, and SIDRA, an Australian gap model, which contain British regression equations relating crash frequency and capacity to geometric parameters.

"RODEL is a British computer application that facilitates the design of flared entries, and is generally thought to be one of the better roundabout-design computer software programs. After the user inputs his traffic performance objective of the average desired delay per vehicle, RODEL lists pairs of possible entry widths and flare lengths to meet the desired objective. The user quickly achieves the optimal design using this and other features of RODEL.

RODEL software contains regression equations relating roundabout capacity to six geometric parameters:

1. Entry width
2. Length of flare
3. Upstream roadway width
4. Diameter
5. Curb return radius
6. Entry angle

R.M. Kimber of the British Transport and Road Research Laboratory developed the regression equations. He used track experiments involving 35 geometric variations and took 11,000 minutes of capacity data at 86 public road sites.⁵

The other two design software applications are not as good as RODEL. The British software application ARCADY does not facilitate the user to quickly select possible entry widths and flare lengths. The Australian software SIDRA produces roundabout capacities that have not been validated by field counts of capacity flows.⁵

Roundabout Acceptance

The superior safety record of modern roundabouts is well known in many other countries besides America. Even though cross intersections appear to be the most dangerous type of intersection, America has held onto the cross intersection design simply because they do not generally recognize the roundabout as an available design alternative. Most American highway engineers have not been exposed to many roundabouts, therefore have little confidence in them. Other Americans question whether drivers who are unfamiliar with this type of intersection can safely adapt to it.⁷ Other countries, especially European countries and countries with British influence such as Australia, have implemented the widespread use of roundabouts. The construction of the first “yield-at-entry” roundabouts in the U.K. in 1956 launched the modern roundabout revolution, in which Western European countries began preferring them over cross intersections. Countries where people drive on the right side of the road, such as the United States, were slower to follow, but are now beginning to catch up.⁷ The U.S. is seeing an increase in roundabout design and construction. Many highway and traffic engineers are becoming roundabout advocates. It seems as though the U.S. is beginning to explore the benefits of roundabouts, and once Americans become familiar with them, the U.S. is sure to see more roundabouts in the future.

References

1. Ourston, Leif, Comparative Safety of Modern Roundabouts and Signalized Cross Intersections, February 26, 1996, http://www.roundabouts.com.
2. Brilon, Werner and Mark Vendehey. Roundabouts – The State of the Art in Germany. ITE Journal, November 1998 p.48 – 54.
3. TD 16/93, Geometric Design of Roundabouts, British Department of Transport, 1993, 4.
4. Ourston & Doctors. Caltrans Endorses Modern Roundabout Interchanges. Caltrans Design Information Bulletin 80. http://www.roundabouts.com.
5. Hall, Gregory A. and Leif Ourston. Roundabouts Increase Interchange Capacity. ITE Journal, December 1997
6. Florida Roundabout Guide, Florida Department of Transportation, March 1996, 55
7. Bared, Joe G. and Leif Ourston. Roundabouts: A Direct Way to Highway Safely. http://www.tfhrc.gov/pubrds/fall95/p94a41.htm

This chapter originally written by Dan Ferster, 1999, as part of a senior project.