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.
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.