Roundabout Intersection 24

Roundabout intersections allow multiple vehicles to enter the intersection simultaneously from any approach when no conflicting vehicle is present in the circulatory roadway. The entry onto a roundabout is controlled by a yield sign. Roundabouts are characterized by the number of circulatory lanes, the number of entry lanes, the central island diameter, the deflection of approaches, flared entries, and splitter islands. The Federal Highway Administration (FHWA) Roundabout Guide (Robinson et al., 2000) can help classify and determine roundabout geometrical dimensions based on the desired operational

characteristics. This guide was developed based on research from Europe and Australia and is currently being updated with U.S. roundabout data. Figure 2-9 provides an example of how a vehicle approaching from the major (a) and minor (b) streets would traverse a roundabout. Note that the figure only demonstrates the movement from one direction for each street, although entry from both directions for both the minor and major street are permitted.

It is important to distinguish roundabouts from the old traffic circles or rotaries. The different is that entering traffic must yield to circulating traffic in a roundabout, while in a rotary, circulating traffic must yield to entering traffic. Furthermore, roundabouts have deflection on the approach legs so that the speed of the vehicle entering the roundabout is sufficiently reduced to minimize the speed differential between the vehicle in the

circulatory roadway and the vehicle on the approach.

Prior to construction of roundabouts in communities not accustomed to them, designers and planners might experience opposition mainly due to unfamiliarity with the design (Retting et al., 2002). Opposition to roundabout intersections can also be attributed to people confusing a roundabout with traffic circles, rotaries, or traffic calming islands (Russell et al., 2002). Agencies should consider providing educational classes and informational sessions regarding roundabouts when implementing them within communities unfamiliar with this design. After implementation of well designed roundabouts, agencies can expect a sufficient decline in the opposition to roundabouts. Once people experience roundabouts, they tend to favor the design (Retting et al., 2002; Russell, 2006).

Roundabouts have the potential to provide improved traffic flow operations at locations with high left-turn volumes, skewed approaches, and conditions with limited queue storage. In general, roundabouts require a shorter sight distance than conventional intersections due to lower speeds on approaches compared to conventional intersections and right turn merge on entry. Traffic leaving roundabouts tends to be more random than at intersections with other types of control. Furthermore, the gaps downstream tend to be shorter but more random and frequent compared to signalized intersections. Thus, roundabouts have the potential to provide more opportunities for side street traffic downstream of the roundabout to enter the major street. Important factors in roundabout design include overall size; entry angles; entry widths; flare length; speed; presence of trucks; pedestrians and bicycles; proper signing; and markings (Johnson and Hange, n.d.).

For multilane roundabouts, special attention to design details such as vehicle path alignment, especially the shortest path, lane widths, and positive guidance to drivers through the use of lane markings, should be carefully considered to achieve a successful roundabout design (rodegerdts et al., 2007).

The two most important driver behavior parameters considered during roundabout design are critical gap and follow-up time. For cities with no prior roundabout experience, these parameters can be assumed to be more conservative than for cities with prior installations of roundabouts. Therefore, when using the guidelines provided by FHWA, longer critical gaps and follow-up times should be assumed than those provided because the FHWA guidelines are based on international research. The critical gaps and follow-up times are longer due to the more conservative nature of U.S. driver behavior on roundabouts compared to driver behavior in other countries (Rodegerdts et al., 2007; Eisenman and List).

The level of service (LOS) for roundabouts should be determined based on the HCM LOS criteria for unsignalized intersections. Control delay should be estimated for each approach separately, not for the intersection as a whole, since it may mask movements with a severe delay. The procedure for determining roundabout LOS can be found in Appendix M (Draft Highway Capacity Manual Chapter 17) of (Rodegerdts, 2007). Roundabouts with heavy traffic are expected to have a higher capacity than roundabouts with light traffic due to drivers accepting shorter gaps in the circulatory flow (Polus et al., 2003).

Microsimulation packages (i.e., VISSIM, Paramics, and others) or macroscopic methods (i.e., RODEL, aaSIDRA, and FHWA methodology) are two other approaches that can be used to determine roundabout capacity (Bared and Edara, 2005; Flannery et al., 1998; Stanek and Milam, 2005). A discussion of these approaches can be found in Appendix A.

To determine roundabout feasibility for a given site, data on the vehicle and pedestrian volumes, and the horizontal and vertical alignment should be considered (Chapman and Benekohal, 2002). Factors that favor roundabout construction include (Chapman and Benekohal, 2002):

¾ Geometric realignment of the approaches

¾ Current alignment is not conducive to the installation of a traffic signal system without geometric improvements

¾ More than four approaches to an intersection exist at a single unsignalized location

Factors that discourage roundabout consideration include (Chapman and Benekohal, 2002; Retting et al., 2002):

¾ Grades through the intersection are greater than four percent ¾ Crest vertical curves with steep approaches are present

¾ Vertical profile cannot be adjusted without a significant expense ¾ Intersection cannot be relocated

¾ Highly unbalanced volumes

¾ Locations where the terrain or right-of-way limit appropriate geometry ¾ Close proximity to persistent bottlenecks

Volumes that favor conversion of a signalized intersection to a roundabout can be found in (Chapman and Benekohal, 2002). Also, there are cases where certain geometric and site characteristics may favor roundabouts over signals. Specific case studies where roundabouts proved to be more efficient than signals can be found in (Johnson and Hange, n.d.). Placing roundabouts within a signalized arterial requires careful analysis, including the possibility of a queue spillback from signalized intersections to the

roundabout, and generally is discouraged above low volumes (Chapman and Benekohal, 2002). Placing roundabouts on arterials with light traffic are easier to justify.

When converting stop-controlled intersections to roundabouts for low and moderate volumes (up to 20,000 veh/day), control delay will be reduced or distributed more fairly between approaches (Flannery et al., 1998). Fair distribution of delay becomes a factor on two-way stop-controlled intersections where the stop-controlled legs experience rapid increases in volume and excessive approach delay even when the average delay for the intersection does not indicate any problems.

Construction of roundabouts at signalized interchanges with high left turn volumes can in some cases reduce costly structure widening and increase capacity (Robinson et al., 2000; Johnson and Hange, n.d.). Roundabouts can also prove to be the most cost-effective solution at the ends of tunnels and bridges, where storage and turning lanes required by a traffic signal would be expensive (Robinson et al., 2000).

Converting stop-controlled intersections to roundabouts reduces delays and vehicle stops. Reduction of the average intersection delay can range from relatively low to significant when converting stop-controlled intersections to roundabouts (Retting et al., 2002; Russell, 2006).

Where an actuated signalized crossing for pedestrians at a roundabout is required, the alternative solution is to locate the crossing downstream of the exit lane. This placement reduces the chance of a queue spilling back to block the circulatory roadway, which is preferred to placing the actuated signalized crosswalk at a splitter island. Placing the crosswalk at a downstream location primarily only affects the exiting vehicles on that particular leg (Rouphail, et al., 2005). The above consideration applies only to signalized pedestrian crossings.