Safety at the roundabout
Thursday, August 30 2012
THIS article on roundabouts continues from the last two weeks (August 9 and 16, 2012).
The safety performance of a roundabout is a product of its design. At roundabouts, vehicles travel in the same direction, eliminating the right-angle and right-turn conflicts associated with traditional intersections. In addition, good roundabout design places a high priority on speed control. Speed control is provided by geometric features, not just by traffic control devices or by the impedance of other traffic. Because of this, speed control can be achieved at all times of day.
Drivers approaching a single-lane roundabout have two basic decisions regarding other users: select the appropriate lane (as applicable) for their intended destination, and yield to those who have the right-of-way.
The frequency of crashes at an intersection is related to the number of conflict points at an intersection, as well as the magnitude of conflicting flows at each conflict point.
A conflict point is a location where the paths of two motor vehicles, or a vehicle and a bicycle or pedestrian path, diverge, merge, or cross each other. Conflicts can arise from both legal and illegal manoeuvres; many of the most serious crashes are caused by failure to observe traffic control devices.
Conflicts can be divided into four basic categories, in which the degree of severity varies, as follows:
(a) Queuing conflicts. These conflicts are caused by a vehicle running into the back of a vehicle queue on an approach. These types of conflicts can occur at the back of a through-movement queue or where left-turning vehicles are queued waiting for gaps. These conflicts are typically the least severe of all conflicts because the collisions involve the most protected parts of the vehicle and the relative speed difference between vehicles is usually less than other conflicts.
(b) Diverging conflicts. These conflicts are caused by the separating of two traffic streams. Examples include right turns diverging from through movements or exiting vehicles diverging from circulating vehicles. If the speed of one movement is significantly different from the other movement, the resulting speed differential increases the risk of a rear-end collision.
(c) Merging conflicts. These conflicts are caused by the joining of two traffic streams. The most common types of crashes due to merging conflicts are side-swipe and rear-end crashes. Merging conflicts can be more severe than diverging conflicts due to the more likely possibility of collisions to the side of the vehicle, which is typically less protected than the front and rear of the vehicle.
(d) Crossing conflicts. These conflicts occur where the paths of two traffic streams intersect. These are the most severe of all conflicts and the most likely to involve injuries or fatalities. Typical crash types are right-angle crashes and head-on crashes.
At roundabouts, crossing manoeuvre is converted into weaving or merging and diverging operations. All traffic, including those turning right or going straight across the roundabout, have equal opportunity as though turning left.
Relative speed is an important factor in addressing these conflicts at intersections. Relative speed is dependent on the absolute speeds of the intersecting vehicles and the angles between them. When the angle of merging is small, the relative speed will also be low. As the relative speed increases, the judgement of drivers regarding time and distance is likely to be more inaccurate, thus increasing the possibility and severity of accidents.
Thus in intersection design, care has to be taken to keep the relative speed low. At roundabouts, the crossing of vehicles is avoided by allowing all vehicles to merge into the streams around the roundabout and then to diverge out to the desired radiating road.
Roundabouts can be advantageous when the number of intersecting roads is between four and seven. Furthermore, the motorist can easily return to the direction originally coming from, through an easy U-turn through the roundabout.
Roundabouts can provide environmental benefits if they reduce vehicle delay and the number and duration of stops compared with an alternative.
Even when there are heavy volumes, vehicles continue to advance slowly in moving queues rather than coming to a complete stop. This may reduce noise and air quality impacts and fuel consumption significantly by reducing the number of acceleration/deceleration cycles and the time spent idling.
The initial design and construction cost of a roundabout can vary significantly depending on the roundabout size, right-of-way impacts, illumination requirements, and other design or aesthetic features that may be desired. Although the initial construction cost may be more, a roundabout can have less operating and maintenance costs than a traffic signal, and the service life of a roundabout is significantly longer.
Roundabouts also provide substantial cost savings to society due to the reduction in crashes, particularly fatal and injury crashes, over their service life.
Compared to signalised intersections, a roundabout does not have signal equipment that requires constant power, periodic light bulb and detection maintenance, and regular signal-timing updates. Roundabouts, however, can have higher landscape maintenance costs depending on the degree of landscaping provided on the central island, splitter islands (for separating entering and exiting traffic, deflecting and slowing entering traffic, and providing a pedestrian refuge), and perimeter.
Drivers sometimes face a confusing situation when they approach a signalised intersection during a power failure, but such failures have minimal temporary effect on roundabouts or any other unsignalised intersections, other than the possible loss of illumination.