During the last few years roundabouts have become more and more popular as a safe alternate way of controlling traffic at major intersections. Yet only a few years ago many North American engineers an...
During the last few years roundabouts have become more and more popular as a safe alternate way of controlling traffic at major intersections. Yet only a few years ago many North American engineers and planners were very resistant to this new traffic control device. European countries as well as Australia have been working on optimising roundabout design for decades. Now, with the introduction in June 2000 of the U. S. Federal Highway Administration’s Roundabouts: An Information Guide, the U. S. and Canada have begun building roundabouts from coast to coast.
Unfortunately, several roadway professionals and designers still confuse a modern roundabout with traffic circles or rotaries. Yet the differences between these two types of traffic control devices are enormous, both in terms of roadway geometry and operational rules. For example, a roundabout has a diameter that is smaller than traffic circles and usually does not exceed 70 metres. Furthermore, roundabouts give priority to those vehicles that are already on the annular roadway as opposed to those entering.
Even in Europe where roundabouts have been implemented for years, giving priority to traffic already proceed- ing around the circle is relatively new. The pioneer in roundabouts, the United Kingdom, changed the rules in the late 1960s, but France changed the rules only in 1983.
In America this concept of giving priority to traffic already on the annular roadway has been difficult for drivers to accept because of the more widespread use of traffic circles. Nevertheless, it’s important to realize that modern roundabouts have a much better capacity for handling traffic then the old rotaries.
Fewer potential accidents. Less stopping
The geometric configuration of roundabouts forces drivers to reduce their speeds as they enter the intersection. This feature makes it easier for drivers to be able to merge onto the roundabout. Roundabouts also facilitate pedestrian crossings, and most importantly reduce the severity of accidents. The number of vehicular conflict points, as well as pedestrian/vehicle and cyclist/vehicle conflicts, is greatly reduced when compared to conventional intersections. Roundabouts have only eight vehicular conflict points, while a standard cross intersection has 32.
Since vehicles approaching a roundabout do not have to stop due to the fact that they are controlled using yield signs, there is also a reduction in delays and unwarranted stops. The potentially continuous flow of vehicles across intersections via a roundabout translates into a reduction in both gas consumption and tailpipe emissions –hence environmental benefits. Traffic signals might run over a full 24-hour period even when their use is not warranted, but roundabouts, due to the way conflicts are managed, allow delays to be reduced at all periods of the day. A key advantage of roundabouts, therefore, is that they allow an optimized traffic flow throughout the day, while traffic lights can cause unnecessary delays during off-peak periods.
Another important benefit of roundabouts is that they allow heavy vehicles like trucks to make large turning movements, most notably left-turns, which are very penalizing to the land requirements for more conventional intersection designs.
Not a miracle solution
However, roundabouts are not a miracle solution. This type of traffic control device should not be implemented on a synchronized arterial in order not to lose the advantage of synchronizing the traffic lights.
Furthermore roundabouts are ineffective at intersections where traffic volumes are heavily unbalanced (ratios of 10:1 or more), and in areas with steep slopes of more than 6%.
In addition, the right-of-way land requirements for this type of traffic control device are often more severe than for traditional intersections, especially in urban environments. In more rural and suburban areas, however, the land requirement for roundabouts can be offset by reducing the number of auxiliary lanes at the approaches to intersections (no left-turn and right-turn lanes), as opposed to more traditional designs.
Design not as easy as it seems
Designing roundabouts is not as easy as with conventional intersections. There is no “one size fits all” recipe that guarantees success. In fact, despite decades of development in Europe, roundabout design is still evolving.
The key to a good design is ensuring that drivers are forced to slow down upon entering the roundabout with the deflection of the approach (see figure 1).
This deflection ensures a safer merge into the roundabout and improves safety for pedestrians and cyclists. However, the design also needs to take into account the local context, topography, and the traffic composition.
The annular circle should be kept as small as pos sible, but ensuring that the design is operable taking into account the number of large trucks and buses. Furthermore, the geometry of the approaches needs to be examined, since many intersections do not offer perfect 90 degree angles and the centre and position of the roundabout need to take this into account.
In addition, the designers need to analyze what type of users access the intersection, in order to determine whether the provision of crosswalks and even bike paths is necessary.
In heavily trafficked areas, particular attention needs to be paid when multi-lane roundabouts are required. This type of design must be carefully developed in order to avoid the overlap of vehicle paths within the circle. Currently, the design and roadway markings of these types of roundabouts are still evolving. Special care must be taken when the approaches and annular circle are large, since this design can encourage higher speeds.
In conclusion, roundabouts are a traffic control device that transportation experts must consider when designing or redesigning an intersection, or when changing how an intersection is managed. While they do have limitations, roundabouts have many advantages when compared to conventional traffic signal controlled intersections, especially when there are a heavy number of left-turn movements.
Will engineering consultants convince planners and designers to add roundabouts to their traffic control tool-box? If we do succeed, we must ensure that they are properly equipped with the knowledge on how to implement roundabouts that are effective and safe in the context of their environment.
Martin Htu, P. Eng. is vice-president transport, western Quebec for GENIVAR, based in Montral. Eric Peissel is director of traffic and transportation planning for GENIVAR, in Montral
Roundabouts: An Information Guide, Federal Highway Administration (FHWA), June 2000.
Synthesis of North American Roundabout Practice. Prepared for Transportation Association of Canada (TAC), September 2006.
Le carrefour giratoire: un mode de gestion diffrent. Ministre des Transports du Qubec, 2002.
Designing roundabouts is not as easy as with conventional intersections. There is no “one size fits all” recipe that guarantees success.