DIRECTOR GENERAL SURFACE POLICY
Geni Bahar, P.Eng.
Thomas Smahel, M.Arch., CCPE
Alison Smiley, Ph.D., CCPE
(Human Factors North Inc.)
March 26, 2009
This report is available upon request only.
Please contact Arif Husain at email@example.com using the title of the report,
"Study of the Environmental, Economic, Safety & Social Benefits of Roundabouts",
to obtain your copy.
The use of roundabouts instead of intersections has gained much attention and momentum over the last 10 years in North America. Approximately 50% of personal injury crashes and 30% of fatalities occur at intersections or are related to intersections (Ontario Ministry of Transportation, 2003). At locations where it is possible to implement roundabouts, this risk can be significantly reduced. In the traffic safety community the considerable benefits in reduction of injuries and improvement in mobility, with positive economic and social benefits, are increasingly recognized. The U.S. Federal Highway Administration (FHWA) has recently developed guidelines for their implementation and is promoting their application. In Canada the Transportation Association of Canada (TAC) is in the process of developing guidelines.
Roundabouts have environmental, economic, and social benefits in addition to those of safety. This report addresses the various impacts of roundabouts through literature review and brief telephone interviews with transportation personnel in Canadian and U.S. jurisdictions. Knowledge gaps are identified and policy recommendations are made.
2 LITERATURE REVIEW
2.1.1 Design Parameters
The success of roundabouts in improving safety, mobility, fuel savings and emissions very much depends on optimal design, traffic flows and vehicle mix. Each roundabout is unique and thus guidelines, which allow the designer flexibility to select the most suitable combination of parameters with respect to design vehicle and turn flow demands, are more appropriate than rigid design standards. The design should always decrease the entry speed to the speed of the circulating vehicles to produce a uniform and lower speed environment.
Roundabouts facilitate the search for pedestrians when crosswalks are placed at least one car length from the circulating lane; both motorists and pedestrians have only one direction to search at a time. A roundabout provides a refuge island for pedestrians thus reducing their exposure to traffic. A roundabout does not need electricity to power signals which results in energy savings and maintenance costs.
2.1.2 Safety, Accident Risk and Accident Severity
Modern well-designed roundabouts have been shown to significantly improve the safety of the intersections when compared with the previous traffic control condition. No other treatments are known to have such a significant positive impact in safety. Overall, total crashes are reduced by 35% and injury crashes by 74%.
In North America, there has not been an extensive study of the safety impact of roundabouts with respect to pedestrians and bicyclists. In the Netherlands, Sweden and in the state of Colorado, roundabouts have been shown to significantly improve pedestrian collisions, while there are conflicting results regarding bicyclist safety.
A recent crash pattern study of modern single and double-lane roundabouts in North America concluded that most crashes were non-injury or property damage only (PDO) collisions. Fifty percent of all crashes in both roundabout types were vehicles colliding with the centre island. Other crashes were of the rear end and sideswipe types. Three-quarters of all crashes occurred at the entrance to the roundabouts. There were no fatal crashes and weather was not a factor.
The key geometric issues related to the crashes included wide entries and minimal deflection entries, no speed limit signs, very short splitter islands, limited visibility of the roundabout, and minimal landscaping of the centre island. It was concluded that low cost treatments and some small changes in geometry can significantly affect the number and types of crashes.
2.1.3 Traffic Flow and Congestion
Modern roundabouts can greatly modify the delays of vehicles when compared with stop-controlled and signalized intersections. The impact varies with the different vehicular and pedestrian demands at the individual entries, as well as the proportion of trucks. The maximum flow rate that can be accommodated at a roundabout entry depends on two factors: the circulatory flow on the roundabout that conflicts with the entry flow, and the geometric elements of the roundabout. Typically, positive benefits in traffic operations are reported with significant decreases in delays at peak morning and afternoon times and an improvement of level of service of at least one to three levels at all approaches.
2.1.4 Speed and Traffic Calming
Although modern roundabouts are not intended as traffic calming devices, when well-designed, they have been shown to effectively reduce speeds at the entrance and around the centre island of mini and typical roundabouts, and along the road segment between two consecutive roundabouts if the distance is less than 300 m.
After the introduction of roundabouts, no speeding was recorded in adjacent areas. At the roundabouts, the overall approach speeds were reduced to 27 to 36 km/h mean speeds, and 31 to 41 km/h 85th percentile speeds, from a prior speed of 50 km/h and higher at all sites.
There were no spillover effects on the road network such as increase or decrease of operating speeds. Rerouting was not observed from any main roads and minor road traffic increased because drivers preferred crossing the main roads at roundabouts.
Noise levels at pedestrian ear level are noticeable quieter in urban roundabouts as compared to stop-controlled intersections. Roundabouts are quieter than other intersection types because traffic flow is improved when speeds reduced. The result is a smoother speed profile and a reduction in the amount of time that vehicles are accelerating and decelerating.
2.1.6 Emissions and Air Quality
Pollutant emission levels such as CO and NOx are significantly lower at roundabouts as compared to stop-controlled and signalized intersections. Emission reductions in turn improve air quality. These benefits are achieved in roundabouts by improving traffic flow which reduces the amount of idle time while stopped.
2.1.7 Fuel Efficiency
Vehicles with combustion engines have higher fuel efficiency when travelling through roundabouts as compared to other intersection types. Combustion engines require far more fuel when idling or accelerating from a stopped position as compared to coasting at a constant speed. The smoother the speed profile through a roundabout, the higher the fuel efficiency.
2.1.8 Social and Regional Benefits
Modern roundabouts, which initially met with scepticism from the public and some professionals, have become the preferred form of intersection. With the introduction of roundabouts, communities and highway agencies have seen the enhancement of the aesthetic and urban design of the adjacent areas to roundabouts, as well as a general calming of traffic leading to improved safety and pedestrian movements. Of particular note is the evidence of economic revitalization effects of roundabouts on nearby businesses. Several communities have also seen the roundabouts as gateway features to their town or local areas.
2.2 Road User Challenges
From a safety, mobility and emissions perspective roundabouts are highly beneficial for light vehicle motorists. However, their continuous flow can present challenges for pedestrians, especially blind pedestrians and bicyclists. Pedestrians can be accommodated easily at single-lane roundabouts, where safety is improved because of the splitter island that allows pedestrians to deal with crossing one direction of traffic at a time. However multi-lane facilities require careful placement of crosswalks and design of signals. Signals should use offset phases to avoid traffic backing up into the circulatory roadway.
Based on cyclist crash statistics, roundabouts are safer than stop and yield-controlled intersections, but multi-lane roundabouts are worse than signalized intersections. Studies show that in multi-lane roundabouts, cyclists benefit from narrow entry points, allowing light vehicles to enter side by side, but forcing trucks to straddle two lanes. Cyclists also benefit from an "S" curve design on the approach, which assists in slowing entering traffic, allowing cyclists to make full use of their lane, and from not having specially marked bicycle lanes.
The longer the wheelbase of a heavy vehicle and the smaller the radius of the roundabout, the greater the difficulty the driver will have negotiating the roundabout. Solutions include an unmarked circulatory roadway with a narrow entry point allowing light vehicles to enter side by side but forcing trucks to straddle two lanes, a truck apron, bypass lanes and at mini-roundabouts, a traversable centre island.
A survey of speciality vehicle (ambulances, school buses, police vehicles, large trucks, etc.) drivers found that knowledge of how to negotiate roundabouts varied greatly from city to city and was best for those drivers who drove more, wore seatbelts and used roundabouts more frequently.
2.3 Cost-benefit Analysis
Roundabouts have benefits of various types: improvements in safety, fuel savings, and reductions in emissions among others. Because the safety benefits in terms of crash savings will be a key component of the cost-benefit analysis, it is important to consider the life cycle of a roundabout (usually 20 years), and compare it with the life cycle of the alternative. Literature on this topic addresses individual benefits with the exception of a theoretical study by Proost and DeGeest (Proost & DeGeest, 2006) which examined a wide range of benefits. It concludes that roundabouts are cost effective, and that a sensitivity analysis indicated that the results were "very robust for changes in accident, time, and infrastructure costs".
The construction cost of a roundabout depends on several factors depending on the setting, that is, urban or rural, the complexity of the changes to the existing site, mostly on the approaches; and maintenance of traffic during construction (Kansas, 2005).
With respect to cost-benefit, motor vehicles with combustion engines are the least efficient when idling or accelerating from a stopped position. Since roundabouts reduce the time spent stopped, a reduction in fuel consumption would be an expected benefit.
On-road studies have found that at roundabouts, pollutant emissions such as CO and NOx are reduced when compared to signalized intersections (Hyden & Varhelyi, 2000; Mandavilli, Russell, & Rys, 2003; Batson, 2008). Replacing an average signalized intersection with a roundabout can be expected to save $500 annually in carbon emissions alone. At the time of writing there are no known metrics to quantify the social cost of NOx emissions.
Maintenance costs of roundabouts can be kept low by designing low-maintenance landscaping.
Five jurisdictions were asked if they calculated cost-benefits of roundabouts. The Region of Waterloo carries out 20-year life cycle costing. Québec does not do cost-benefit analyses, and sees safety benefits as the prime reason for roundabout installation. Nova Scotia does not do cost-benefit analyses, but has identified situations in which roundabouts are a less expensive solution than the standard alternative. The City of Calgary guidelines indicate that the retrofitting of roundabouts, when the roads were already built for signalized intersections, is challenging to justify, even with the safety, environmental and life-cycle cost savings. On the other hand, roundabout construction was found to be preferred on new streets within new developments or where there is the need for reconstruction of the intersection. (Sargeant & Vanderputten, 2009).
The Virginia Department of Transport (DOT) indicate that initial construction costs are often the deciding factor, which gives advantage to the signalized intersection, but that a 20-year cost-benefit analysis would show that the roundabout is the preferred choice.
2.4 Design and Policy Guidelines
2.4.1 Design Guidelines
Roundabout design requires a shift in thinking; it needs flexibility and a proper design cannot be achieved by selecting standard values for design parameters. This holistic approach to design fits well with the design approach advocated by TAC in Canada. The best combination of parameter values are selected for each site and approach to a roundabout. Some provinces and regions have developed their own guidelines since there is not a national guide for roundabouts in Canada; several U.S. states have developed supplementary guidelines and policies which work hand-in-hand with the FHWA Roundabout Guide (2000).
2.4.2 Policy Guidelines
Six jurisdictions were asked about policies with regard to roundabout implementation. British Columbia and Virginia formally require that a roundabout be looked at as the first option for an intersection; Québec also does this but there is no formal policy. Nova Scotia considers roundabouts on an ad hoc basis. California sets down required design standards. Arizona requires a process for obtaining community approval.
3 KNOWLEDGE GAPS
There is indication that roundabouts work well along corridors where intersections are signal controlled. There is a need to better understand the distances between consecutive intersections, and traffic flows to provide traffic engineers with the tools to developed optimum coordination and minimum delay.
No guidelines were found on illumination of roundabouts. These are needed to guide the optimum location of poles and height of luminaries, with particular focus on avoiding the locations where run-off-the road crashes happen, and where pedestrians cross the roadway.
In North America, there has not been an extensive study of the safety impact of roundabouts with respect to pedestrians and bicyclists, and how best to provide access and mobility to non-motorized road users. Given the lack of North American experience, research to document potential pitfalls and practices of using signals to provide access for pedestrians at roundabouts is proposed. More effort is required to solve problems presented by roundabouts to pedestrians with disabilities.
With respect to trucks, various cross-sections are used for the circulatory roadway, each with a different impact on the tractor, on the trailer and on the truck driver's sensation of lateral acceleration. Research is needed to determine which is most effective for preventing truck overturns.
Pollutant emissions and fuel consumption has been measured at small, single-lane roundabouts and compared to similar size stop-controlled and signalized intersections. These smaller intersection types tend to have short queue lengths and idle times. Little is known about what improvements can be expected converting wide cross-section intersections into two-lane roundabouts. With respect to cost-benefit calculations, at the time of writing there are no known metrics to quantify the social cost of NOx emissions.
There is a need to develop traffic operations and safety warrants or guidelines to assist professionals in identifying intersections that would be candidates to convert to roundabouts based on their past performance or when in the planning stages. Similar to the existing warrants and guidelines, these need to be provided for stop-controlled and signalized intersection conversion. These should be based on safety performance models developed for all types of traffic control and on cost-benefit analysis including a broad range of factors.
Guidelines for the appropriate landscaping of the centre island and the surrounding sidewalk and split-islands are needed with focus on maintenance, visibility and sight lines, and seasonal issues.
The prevalence of roundabouts is growing exponentially in North America as awareness of their considerable benefits grows in the transportation community. Roundabouts have been demonstrated to have substantial benefits for drivers of light vehicles with respect to greatly reduced injury and fatality collisions. Single-lane roundabouts also improve safety for pedestrians and cyclists, but multi-lane roundabouts require careful location of crossings and design of signals for pedestrians, and slowing of entering traffic through design and marking for cyclists. Drivers of heavy vehicles need special design considerations to ensure turning can be accomplished without negating the traffic-calming features of roundabouts. Roundabouts also have been demonstrated to save fuel and to reduce noise and emissions. Roundabouts will often be less cost beneficial than traditional intersections if only initial construction costs are considered. However, a 20-year analysis of construction and maintenance costs and benefits will often show a benefit for roundabouts, including injury and fatality costs, let alone costs related to fuel savings and lower emissions increases the cost benefit.
5 POLICY RECOMMENDATIONS
Given the considerable benefits of roundabouts in terms of greatly improved safety, increased mobility, reduced fuel use and emissions as well as social benefits, Canadian jurisdictions should consider a proactive policy, such as that used by British Columbia and Virginia, formally requiring that a roundabout be looked at as a first option for an intersection. The cost-benefit analyses should go well beyond initial construction costs and consider in addition environmental, economic and safety costs over a 20 year life-cycle period. As cost-benefit analysis experience accumulates, policies can be made more specific with respect to the best types of location at which to implement roundabouts.
There are a number of guidelines available, appropriate to use on Canadian roadways. The Transportation Association of Canada (TAC) is in the process of developing national guidelines. However, some Canadian jurisdictions have already developed their own, drawing on European, Australian and U.S. sources. These can be adopted until the TAC guidelines are completed.
Recognizing the challenges faced by various road user groups, the policy should ensure that the design guidelines used address the needs of pedestrians with various types of impairments, bicyclists and heavy and speciality vehicle users, depending on the prevalence of these various user classes. Recognizing the novelty of roundabouts in many jurisdictions, this policy should also consider the need for communication with and education of the public in advance as described in the Kansas (2005) design guidelines.