Roundabouts: Part 3 Quiz

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Designing a roundabout is a process of determining the optimal balance between safety provisions, operational performance, and large vehicle accommodation.

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Before the details of the geometry are defined, three fundamental elements must be determined in the preliminary design stage:

1. The optimal roundabout size;
2. The optimal position; and
3. The optimal alignment and arrangement of approach legs.

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Note that double-lane roundabout design is significantly different from single-lane roundabout design, and many of the techniques used in single-lane roundabout design do not directly transfer to double-lane design.

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Because it has profound impacts on safety, achieving appropriate vehicular speeds through the roundabout is the most critical design objective. A well-designed roundabout reduces the relative speeds between conflicting traffic streams by requiring vehicles to negotiate the roundabout along a curved path.

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International studies have shown that increasing the vehicle path curvature decreases the relative speed between entering and circulating vehicles and thus usually results in decreases in the entering-circulating and exiting-circulating vehicle crash rates. However, at multilane roundabouts, increasing vehicle path curvature creates greater side friction between adjacent traffic streams and can result in more vehicles cutting across lanes and higher potential for sideswipe crashes (2). Thus, for each roundabout, there exists an optimum design speed to minimize crashes.

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It is important when designing the roundabout geometry that the radius of the entry path (i.e., as the vehicle curves to the right through entry geometry) not be significantly larger than the circulatory path radius.

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Along with overall reductions in speed, speed consistency can help to minimize the crash rate and severity between conflicting streams of vehicles.

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The exit radius should not be less than entry path radius or the circular path radius in order to minimize loss-of-control crashes. 

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The exit radius should not be less than entry path radius or the circular path radius in order to minimize loss-of-control crashes. 

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Another important factor determining a roundabout’s layout is the need to accommodate the largest motorized vehicle likely to use the intersection. The turning path requirements of this vehicle, termed hereafter the design vehicle, will dictate many of the roundabout’s dimensions.

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Like the motorized design vehicle, the design criteria of nonmotorized potential roundabout users (bicyclists, pedestrians, skaters, wheelchair users, strollers, etc.) should be considered when developing many of the geometric elements of a roundabout design. These users span a wide range of ages and abilities that can have a significant effect on the design of a facility.

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The inscribed circle diameter is the distance across the circle inscribed by the outer curb (or edge) of the circulatory roadway.

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Entry width is the largest determinant of a roundabout’s capacity. The capacity of an approach is not dependent merely on the  number of entering lanes, but on the total width of the entry.

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Stopping sight distance is the distance along a roadway required for a driver to perceive and react to an object in the roadway and to brake to a complete stop before reaching that object. 

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Intersection sight distance is the distance required for a driver without the right of way to perceive and react to the presence of conflicting vehicles. Intersection sight distance is achieved through the establishment of adequate sight lines that allow a driver to see and safely react to potentially conflicting vehicles. At roundabouts, the only locations requiring evaluation of intersection sight distance are the entries.

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It is generally not desirable to locate roundabouts in locations where grades through the intersection are greater than 4%. The installation of roundabouts on roadways with grades lower than three percent is generally not problematic.

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In general, right-turn bypass lanes (or right-turn slip lanes) should be avoided, especially in urban areas with bicycle and pedestrian activity. The entries and exits of bypass lanes can increase conflicts with bicyclists. The generally higher speeds of bypass lanes and the lower expectation of drivers to stop increases the risk of collisions with pedestrians. However, in locations with minimal pedestrian and bicycle activity, right-turn bypass lanes can be used to improve capacity where there is heavy right turning traffic.

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Pavement word markings are less effective in rainy or especially snowy climates.

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In addition, bike lane markings within the circulatory roadway are not recommended. The additional width of a bike lane within the circulatory roadway increases vehicular speed and increases the probability of motor vehicle-cyclist crashes. Bicyclists should circulate with other vehicles, travel through the roundabout as a pedestrian on the sidewalk, or use a separate shared-use pedestrian and bicycle facility where provided.

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For roundabouts in suburban settings, illumination is recommended. For safety reasons, illumination is necessary when:

• One or more approaches are illuminated.
• An illuminated area in the vicinity can distract the driver’s view.
• Heavy nighttime traffic is anticipated.

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It is highly desirable to detour traffic for construction of a roundabout. This will significantly reduce the construction time and cost and will increase the safety of the construction personnel. If it is not possible to detour all approaches, detour as many approaches as possible and stage the remainder of the construction as follows:

1. Install and cover proposed signing.
2. Construct outside widening if applicable.
3. Reconstruct approaches if applicable.
4. Construct splitter islands and delineate the central island. At this point the signs should be uncovered and the intersection should operate as a roundabout.
5. Finish construction of the central island.

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If fountains or monuments are being considered for the central island, they must be designed in a way that will enable proper viewing from the perimeter of the roundabout. In addition, they must be located and designed to minimize the possibility of impact from an errant vehicle.

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Roundabouts should not be planned for signalization or metering unless unexpected demand dictates this need after installation.

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Closely spaced roundabouts may improve safety by “calming” the traffic on the major road. Drivers may be reluctant to accelerate to the expected speed on the arterial if they are also required to slow again for the next close roundabout. This may benefit nearby residents.

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A raindrop central island can be considered to be a circular shape blocked at one end. In this configuration, a driver wanting to make a U-turn has to drive around both raindrop-shaped central islands. This configuration has an additional advantage in that it makes wrong-way turns into the off-ramps more difficult.