Roundabout Design Basics
These statements are very general but follow the process of design.

Horizontal Design: As modern roundabouts replace cross intersections, right angle crashes become less severe and less frequent, and left turning crashes do not occur. Rear-end crashes become less frequent because roundabouts have less queuing. To provide good vehicle path deflection, an important safety design feature that slows traffic on entry, the approach roads are aimed toward left of the central island. They veer back to the right just before the yield lines. Roundabouts are typically designed for speeds from 20 to 25 mph.

Once the horizontal alignment and location of the center island are determined the geometry is checked by turning templates to see if the design vehicle (usually a truck or a bus)can travel through the roundabout without leaving the pavement. Changes are made to the entries, circulatory lane, and exits as needed.

Vertical design follows horizontal design to ensure that approach roads enter and exit the roundabout at optimal grades and smoothness of curves. Care is taken to design the roundabout plateau with a cross slope of 4% or less.

This site recommends using a crowned cross-section around the circulatory lane that slopes 2/3 into the circle and 1/3 away from the circle. This has been found to help with multi-lane roundabouts in separating left-turn traffic from straight or right-turn traffic. It is especially useful when roundabouts are added to streets with some steep grades on the approaches as it reduces the potential for trucks to overturn on the low side of the circle. This opinion has been formed from years of driving in roundabouts in France, Switzerland, New Zealand, Australia and England. The Brits have more experience and data to back up their designs than the rest of the world combined and have found this vertical design to move more traffic while causing fewer accidents than having a straight 2% slope away from the central island as is common in France and parts of the USA.

The capacity of the roundabout is analyzed based on the design parameters. Modern roundabouts are designed for minimum speed and size. The number of entry lanes depends on the future capacity needs. Generally, single-lane roundabouts experience a lower number of accidents than multi-lane roundabouts.

Where to Use Roundabouts

Appropriate conditions for roundabout installation:

• Locations with high delays
• Locations where traffic signals are not warranted
• Four-way stop intersections
• Intersections with more than four legs
• Intersections with high left-turn flows
• Intersections with unusual geometry
• Intersections with changing traffic patterns
• Locations where storage capacities for signalized intersections are restricted
• Intersections that are important from an urban design or visual point of view

The following conditions require special design attention and hiring an experienced designer:

• Locations where there is insufficient space for an acceptable outside diameter.
• Locations where it would be difficult to provide flat (2% or less) entries into the roundabout.
• Maximum grade greater than 4% around the circle,
• A high number of pedestrians, a high percentage of large trucks, intersection junction at the top or bottom of a grade, and the close proximity of adjacent signals..

The following design topics can be used as a checklist when designing roundabouts.

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Design Checklist

• Feasibility analysis
• Preliminary design
• Final design

Design for Pedestrians and Bicycles

• Crosswalk location
• Flashers
• Marked versus unmarked crosswalks
• Alternate pavement
• Pedestrian tunnels--Swedish style
• Bike Lanes or not in roundabout
• Pedestrian FAQ :(in Bainbridge Island School Roundabout)

Minimum Diameter for Trucks

• AutoTurn
• Truck aprons
• Cross slope effects

Capacity Analysis

• VISSIM:(Verkehrs Im Stadt SIM) is now recommended by RoundaboutsUSA for analyzing roundabout designs and traffic flows. What you see is what you get. How can you beat that? This model is very useful for public presentations. You can model the effect of larger splitter islands width near the entry.

• SIDRA Software: Gap acceptance method for analysis. Utilize change to the environmental factor to 1.2 to use in the USA. May require additional calibration to adjust for drivers in the USA. To purchase SIDRA, contact Akcelik & Associates at the following address: SIDRA Software

  Circulation Lane Widths

  Acceptable Grades

  Cross Slopes and Drainage

  • Minimum Slopes
  • Crowns

  Lighting Good street lighting is a standard safety element of modern roundabout design. Motorists approaching at night must see that the intersection has a central island and that one can no longer drive straight through the intersection. Good street lighting is needed so that cyclists, motorcyclists, and pedestrians can be seen in the roundabout an don the approaches at night. For this reason additional street lights should be installed at roundabouts and on the approaches back a distance of over 150 feet from the yield lines. The lighting design should provide at least 1.9 footcandles of horizontal luminance. Street lights evenly spaced in a ring around the outside of roundabouts and along the approaches to roundabouts works the best.

  Landscaping The central islands of roundabouts allow for attractive landscaping. To provide adequate stopping sight distance for circulating traffic, the outer margins of the central islands must have low ground covers. The central portions of these islands will be available for objects of any height, including trees, walls, and public art. The Colorado towns of Vail and Avon, and the cities of Addison Park, Texas and Las Vegas, Nevada offer good examples of beautiful community entry statements created by roundabouts.

  Signing and Striping
  Uniformity, MUTCD, Europe versus USA:

  • Yield ahead sign
  • YIELD signs
  • Chevron sign
  • ONE-WAY sign
  • Object sign
  • Crosswalk Warning sign
  • Circular Roundabout Warning sign
  • Speed Advisory sign
  • Street sign
  • Directional or route sign

  Center Island Design
  Design Elements:

  • Truck aprons
  • Curb types
  • Landscaping
  • Lighting design

  Cost Estimates
  

  • Right of way
  • Pavement
  • Landscaping
  • Lighting
  • Grading and drainage

  Definitions
  

  • Inscribed circle diameter (ICD)
  • Entry flare
  • Deflection
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  Justification: 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
  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.

  Roundabouts
  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.
  

  Roundabout "Warrants"
  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.

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