Roundabouts

By now, you probably are aware that the 2010 Highway Capacity Manual (HCM) includes an analysis method for analyzing roundabouts.  Unlike the roundabout analysis method within the HCM 2000, the HCM 2010 includes a robust method for analyzing both one and two lane roundabouts with or without slip ramps.  The method was largely developed using field data collected at 31 locations within the U.S.  NCHRP Report 572 provides an overview of the roundabout and the data collected.

As engineers or planners, many of us often wonder how a particular parameter can influence the results of a particular analysis scenario.  Many of us rely on the default parameters and tend not to change them unless there is good reason.  Two of the key parameters of the HCM 2010 methodology, follow-up and critical headway, play an important role determining the capacity and delay of roundabouts.  In Chapter 33 of the HCM 2010, users are encouraged to calibrate headways to those observed in the field due to the significance that these parameters have on the calculations of capacity and delay.

To shed some light on the significance of these factors, today’s post provides a comparison of the default headway values to those collected within the field.  During one of our Synchro Studio Training Courses, one of the attendees from Reid-Middleton, a well-known consultant specializing in roundabout design, was in attendance.  They were kind enough to share field data from a few recently constructed roundabouts located in the northwest.

The HCM 2010 roundabout method assigns a follow-up and critical headway based upon the geometric configuration of the roundabout.  The default headway values are considered somewhat high for locations that have drivers familiar with traversing roundabouts.  Generally speaking, the more familiar drivers are with roundabouts, lower headway values will be observed in the field.

This was true at each of the six roundabout locations provided by Reid-Middleton.  Table 1 includes a summary of the default headway values based on the HCM 2010 and those calculated based on field data.  Note the slight differences between the values.

An operational analysis was conducted using both the default and field measured headway values at each of the six roundabouts.  Table 2 depicts the turning volumes at each intersection, while Table 3 depicts the average delay by approach and the overall intersection.  Although the headway values have differences of less than one second, the differences in delay range between 6 seconds to over 50 seconds.

When analyzing roundabouts using HCM 2010 methodology, remember to consider the importance of calibrating the headway factors.  One of the benefits of Synchro is the ease of analyzing different types of intersections in the same file without having to recode the intersection.