HCM 6th Edition: Roundabout Calculation Changes

The 6^th Edition of the Highway Capacity Manual (HCM) is out and we (and others) would characterize it as an update as opposed to an overhaul. While this new manual does contain new and interesting items, we’ll focus on roundabouts for this article.

HCM2010 was ground-breaking in the U.S. for providing a new methodology to analyze roundabouts, giving all practitioners a common starting ground. The 6^th Edition leaves those basic formulas the same. The major change is instead a decrease of time in the critical headway and follow-up headway.

Translated, this means drivers are becoming more comfortable driving through roundabouts by:

• Having less hesitation entering a roundabout
• Using smaller gaps to enter the roundabout
• Following closer to other vehicles entering the roundabout and circulating within the roundabout

The end result is the HCM 6^th Edition assumes a much higher capacity for roundabouts than previously estimated. There are two sample problems in the HCM2010 and 6^th Edition that demonstrate this increase in capacity. Each sample problem uses the same volume inputs and roundabout characteristics, but the outcomes are as follows:

• Single Lane Roundabout Sample Problem.
  HCM2010 – 34.8 seconds per vehicle intersection control delay
  HCM 6^th Edition – 17.5 seconds per vehicle intersection control delay

• Multi-Lane Roundabout Sample Problem.
  HCM2010 – 19.7 seconds per vehicle intersection control delay
  HCM 6^th Edition – 12.3 seconds per vehicle intersection control delay

That’s about a 50% and 38% improvement for the single-lane and multi-lane roundabouts, respectively. We’ve noticed improved driving around roundabouts in our own work, so this is a welcome change that meets with our observations.

Here are the exact factors that changed from the HCM2010 to the 6^th Edition:

[Dear Readers – Please note that the labels for the 2 (Right Lane) and 2 (Left Lane) in the above chart were initially mislabeled when the article was released, with the “Right” and “left” switched. The table above has been updated to correctly reflect the name of the label of the column. The corrected labels are listed in red. The data for the critical headway and follow-up headway has not changed.]

We’ve already updated our Roundabout Capacity Analysis spreadsheet to reflect this new information. In the meantime, here’s how you can manually adjust the parameters for PTV Vistro and Trafficware Synchro.

Trafficware Synchro Studio

PTV Vistro

Can’t wait for the software updates? Check out our own Roundabout Analysis spreadsheet. We’ve updated this easy to use and adjust spreadsheet to the HCM 6^th Edition methodology. Once purchased, it’s your to edit, customize, and make your own. Check it out here.