Speeds through the roundabout
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.
Speed profiles
Exhibit 6-3 shows the operating speeds of typical vehicles approaching and negotiating a roundabout. Approach speeds of 40, 55, and 70 km/h (25, 35, and 45 mph, respectively) about 100 m (325 ft) from the center of the roundabout are shown. Deceleration begins before this time, with circulating drivers operating at approximately the same speed on the roundabout. The relatively uniform negotiation speed of all drivers on the roundabout means that drivers are able to more easily choose their desired paths in a safe and efficient manner.
Design speed
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.
Recommended maximum entry design speeds for roundabouts at various intersection site categories are provided in Exhibit 6-4.
Vehicle paths
To determine the speed of a roundabout, the fastest path allowed by the geometry is drawn. This is the smoothest, flattest path possible for a single vehicle, in the absence of other traffic and ignoring all lane markings, traversing through the entry, around the central island, and out the exit. Usually the fastest possible path is the through movement, but in some cases it may be a right turn movement.
A vehicle is assumed to be 2 m (6 ft) wide and to maintain a minimum clearance of 0.5 m (2 ft) from a roadway centerline or concrete curb and flush with a painted edge line (2). Thus the centerline of the vehicle path is drawn with the following distances to the particular geometric features:
- 1.5 m (5 ft) from a concrete curb,
- 1.5 m (5 ft) from a roadway centerline, and
- 1.0 m (3 ft) from a painted edge line.
Exhibits 6-5 and 6-6 illustrate the construction of the fastest vehicle paths at a single-lane roundabout and at a double-lane roundabout, respectively.
As shown in Exhibits 6-5 and 6-6, the fastest path for the through movement is a series of reverse curves (i.e., a curve to the right, followed by a curve to the left, followed by a curve to the right). When drawing the path, a short length of tangent should be drawn between consecutive curves to account for the time it takes for a driver to turn the steering wheel. It may be initially better to draw the path freehand, rather than using drafting templates or a computer-aided design (CAD) program. The freehand technique may provide a more natural representation of the way a driver negotiates the roundabout, with smooth transitions connecting curves and tangents. Having sketched the fastest path, the designer can then measure the minimum radii using suitable curve templates or by replicating the path in CAD and using it to determine the radii.
The design speed of the roundabout is determined from the smallest radius along the fastest allowable path. The smallest radius usually occurs on the circulatory roadway as the vehicle curves to the left around the central island. However, 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.
The fastest path should be drawn for all approaches of the roundabout. Because the construction of the fastest path is a subjective process requiring a certain amount of personal judgment, it may be advisable to obtain a second opinion.
Speed-curve relationship
The relationship between travel speed and horizontal curvature is documented in the American Association of State Highway and Transportation Officials’ document, A Policy on Geometric Design of Highways and Streets, commonly known as the Green Book (4). Equation 6-1 can be used to calculate the design speed for a given travel path radius.
Superelevation values are usually assumed to be +0.02 for entry and exit curves and -0.02 for curves around the central island. For more details related to superelevation design, see Section 6.3.11.
Values for side friction factor can be determined in accordance with the AASHTO relation for curves at intersections (see 1994 AASHTO Figure III-19 (4)). The coefficient of friction between a vehicle’s tires and the pavement varies with the vehicle’s speed, as shown in Exhibits 6-8 and 6-9 for metric and U.S. customary units, respectively.
Speed consistency
In addition to achieving an appropriate design speed for the fastest movements, another important objective is to achieve consistent speeds for all movements. Along with overall reductions in speed, speed consistency can help to minimize the crash rate and severity between conflicting streams of vehicles. It also simplifies the task of merging into the conflicting traffic stream, minimizing critical gaps, thus optimizing entry capacity. This principle has two implications:
- The relative speeds between consecutive geometric elements should be minimized; and
- The relative speeds between conflicting traffic streams should be minimized.
As shown in Exhibit 6-12, five critical path radii must be checked for each approach. R1 , the entry path radius, is the minimum radius on the fastest through path prior to the yield line. R2 , the circulating path radius, is the minimum radius on the fastest through path around the central island. R3 , the exit path radius, is the minimum radius on the fastest through path into the exit. R4 , the left-turn path radius, is the minimum radius on the path of the conflicting left-turn movement. R5 , the right-turn path radius, is the minimum radius on the fastest path of a right-turning vehicle. It is important to note that these vehicular path radii are not the same as the curb radii. First the basic curb geometry is laid out, and then the vehicle paths are drawn in accordance with the procedures.
On the fastest path, it is desirable for R1 to be smaller than R2 , which in turn should be smaller than R3 . This ensures that speeds will be reduced to their lowest level at the roundabout entry and will thereby reduce the likelihood of loss-of-control crashes. It also helps to reduce the speed differential between entering and circulating traffic, thereby reducing the entering-circulating vehicle crash rate. However, in some cases it may not be possible to achieve an R1 value less than R2 within given rightof- way or topographic constraints. In such cases, it is acceptable for R1 to be greater than R2 , provided the relative difference in speeds is less than 20 km/h (12 mph) and preferably less than 10 km/h (6 mph).
At single-lane roundabouts, it is relatively simple to reduce the value of R1 . The curb radius at the entry can be reduced or the alignment of the approach can be shifted further to the left to achieve a slower entry speed (with the potential for higher exit speeds that may put pedestrians at risk). However, at double-lane roundabouts, it is generally more difficult as overly small entry curves can cause the natural path of adjacent traffic streams to overlap. Path overlap happens when the geometry leads a vehicle in the left approach lane to naturally sweep across the right approach lane just before the approach line to avoid the central island. It may also happen within the circulatory roadway when a vehicle entering from the righthand lane naturally cuts across the left side of the circulatory roadway close to the central island. When path overlap occurs at double-lane roundabouts, it may reduce capacity and increase crash risk. Therefore, care must be taken when designing double-lane roundabouts to achieve ideal values for R1 , R2, and R3 . Section 6.4 provides further guidance on eliminating path overlap at double-lane roundabouts.
The exit radius, R3 , should not be less than R1 or R2 in order to minimize loss-of-control crashes. At single-lane roundabouts with pedestrian activity, exit radii may still be small (the same or slightly larger than R2) in order to minimize exit speeds. However, at double-lane roundabouts, additional care must be taken to minimize the likelihood of exiting path overlap. Exit path overlap can occur at the exit when a vehicle on the left side of the circulatory roadway (next to the central island) exits into the right-hand exit lane. Where no pedestrians are expected, the exit radii should be just large enough to minimize the likelihood of exiting path overlap. Where pedestrians are present, tighter exit curvature may be necessary to ensure sufficiently low speeds at the downstream pedestrian crossing.
The radius of the conflicting left-turn movement, R4 , must be evaluated in order to ensure that the maximum speed differential between entering and circulating traffic is no more than 20 km/h (12 mph). The left-turn movement is the critical traffic stream because it has the lowest circulating speed. Large differentials between entry and circulating speeds may result in an increase in single-vehicle crashes due to loss of control. Generally, R4 can be determined by adding 1.5 m (5 ft) to the central island radius. Based on this assumption, Exhibits 6-13 and 6-14 show approximate R4 values and corresponding maximum R1 values for various inscribed circle diameters in metric and U.S. customary units, respectively.
Finally, the radius of the fastest possible right-turn path, R5 , is evaluated. Like R1 , the right-turn radius should have a design speed at or below the maximum design speed of the roundabout and no more than 20 km/h (12 mph) above the conflicting R4 design speed.
