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Roundabouts vs. Signals and Stop Signs
Three general questions must be answered to justify a roundabout as the most appropriate form of control at any intersection:
- Will a roundabout be expected to perform better than other alternative control modes? In other words, will it reduce delay, improve safety or solve some other operational problem?
- Are there factors present to suggest that a roundabout would be a more appropriate control, even if delays with a roundabout are slightly higher?
- If any contraindicating factors exist, can they be resolved satisfactorily?
If these questions may be answered favorably, then a roundabout should be considered as a logical intersection choice.
Roundabout controlled intersections can efficiently service traffic with decreased delay and greater efficiency than traffic signals. This is particularly true where traffic volumes entering the roundabout are nearly balanced on all legs and where there are a high number of left turning vehicles.
Traffic signals cause unnecessary delay for many reasons:
- The need to provide a minimum green time to each movement in every cycle creates time intervals in which no vehicles are entering the intersection.
- The need to provide for the most critical of two or more movements that proceed simultaneously results in an ineffective use of green time by non-critical movements.
- The "lost time" associated with startup and termination of a green phase detracts further from the amount of time that is available for moving traffic.
- Left turns that take place from shared lanes impede the other movements in the shared lanes unnecessarily. This results in a very inefficient utilization of the available roadway space.
- Heavy left turns, even from exclusive lanes, require dedicated phases that rob time from the major movements and increase the total time lost due to startup and termination of traffic movements.
- Signals are mechanical devices that not only require maintenance but also periodically malfunction. They are also dependent upon electrical power and do not, therefore, provide any control during power failures.
- Many signal violations occur at higher speeds so that the severity of accidents is often high.
- Permitted left turns and right turns on red introduce additional conflicts.
Two-Way Stop Control (TWSC)
TWSC can accommodate low traffic volumes with much less delay than traffic signals, but this control mode favors the major street (unstopped) movements at the expense of the minor street (stopped) movement. When the major street traffic volumes are heavy (typically 1400 vph or more) there is little or no opportunity for cross street access. This places a definite limit on the application of TWSC. Even when TWSC capacity is not exceeded, there is often public pressure to install signals at TWSC intersections.
All-Way Stop Control (AWSC)
AWSC treats the cross street movements more favorably, without the wasted time associated with traffic signals. However, the rate at which vehicles may enter an intersection (i.e. headway) under AWSC is relatively low and, therefore, the total intersection capacity is somewhat limited.
A roundabout overcomes all of these disadvantages. There is no sequential assignment of right-of-way and therefore no wasted time. Left turns are not subordinated to through traffic. Vehicles enter under yield control instead of stop control and therefore have lower headways and higher capacities. There are no electrical components to malfunction.
Roundabouts, on the other hand, have their own limitations:
- Steady-state entry headways are shorter at traffic signals because of the positive assignment of right-of-way. By using long cycle times to minimize the effects of startup lost time, it is possible under most conditions to achieve higher approach capacities.
- For very low-volume applications, TWSC and AWSC are easier and less expensive to implement.
- Since roundabout operation is not periodic, it is not possible to coordinate the operation of roundabouts on an arterial route to provide smooth progression for arterial flows.
- Roundabouts offer the least positive form of control. Each vehicle entering the intersection must yield to all traffic that has already entered, but they still experience less accidents than a comparable signal or stop control.
- Roundabouts impose a new form of traffic control that is not familiar to motorists in the USA, but experience has found that drivers learn quickly how to drive in a roundabout.
Therefore, roundabouts are not the solution to all traffic problems at all locations. Careful study is required to identify the most appropriate control method at any given location. The studies required to justify the installation of traffic signal control and all-way stop control are based on the warrants and requirements set forth in the Manual of Uniform Traffic Control Devices (MUTCD). A Modern Roundabout should be considered as an alternative traffic control to traffic signals and stop sign control.
Before deciding on a traffic signal, other types of traffic control devices should be considered. Many of these will have benefits in terms of operations as they offer less restrictive movement of the vehicles through the intersection. At the same time, each of them will also offer some safety benefits over a signalized intersection. The 1998 Texas DOT document "Traffic Signal Warrants" states that the objective of an intersection review is to "utilize the least restrictive form of traffic control that produces safe and efficient vehicle and pedestrian operation. These other forms of right-of-way control should be considered even if the intersection meets one or more of the traffic signal warrants.
Authorities in Western Australia consider a traffic signal as their 'last alternative' when considering improvements to an intersection. Among the different types of 'less restrictive' traffic control devices, traffic roundabouts have been proposed by a growing number of municipal and state authorities. Traffic roundabouts offer many significant safety benefits over signalized intersections and have the same capacity to handle a significant volume of traffic (up to 7000 entering vehicles per hour - 60,000 vehicles/day). Traffic roundabouts have fewer conflict points than a signalized intersection and tend to have accidents of a less severe nature because there are no opposing movements. In particular, traffic roundabouts can be considered as an alternative to a signalized intersection when there are a high number of right angle, head on, left turn/through or U-turn accidents or intersections with an unusually high accident severity ratio (Florida DOT 1996).
The following jurisdictions have formally recommended traffic roundabouts as an alternative traffic control device to a signalized intersection, given that the same careful operational and safety investigations are carried out:
- New York State Department of Transportation
- Virginia Department of Transportation
- Maryland Department of Transportation
- Oregon Department of Transportation
- Florida Department of Transportation
- California Department of Transportation
- Federal Highway Administration (FHWA)
FHWA has recently published a comprehensive report on roundabouts entitled "Roundabouts: An Informational Guide" (FHWA 2000). This report contains a section on warrants for roundabouts.