HCM 6th Edition: Roundabout Calculation Changes

The 6th 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 6th 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 6th Edition assumes a much higher capacity for roundabouts than previously estimated. There are two sample problems in the HCM2010 and 6th 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 6th Edition – 17.5 seconds per vehicle intersection control delay
  • Multi-Lane Roundabout Sample Problem.
    HCM2010 – 19.7 seconds per vehicle intersection control delay
    HCM 6th 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 6th Edition:

Capacity Calibration

HCM2010

HCM 6th Edition

Approach
Lanes

Circulating Lanes Critical Headway (s) Follow-Up Headway (s) Critical Headway (s)

Follow-Up Headway (s)

1

1 5.193 3.186 4.990

2.609

2

1 5.193 3.186 4.550

2.536

1

2

4.113 3.186 4.320 2.536

2 (Left Lane)

2 4.293 3.186 4.650

2.667

2 (Right Lane)

2 4.113 3.186 4.320

2.536

Yield Right
By-Pass Lane

1 5.193 3.186 4.990

2.609

Yield Right
By-Pass Lane
2 4.113 3.186 4.320

2.536

[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

screenshot_1

PTV Vistro

screenshot_2

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 6th Edition methodology. Once purchased, it’s your to edit, customize, and make your own. Check it out here.

Please note: I reserve the right to delete comments that are offensive or off-topic.

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4 thoughts on “HCM 6th Edition: Roundabout Calculation Changes

  1. Dear Mike

    The HCM Edition 6 roundabout capacity model aplication is already available in the SIDRA INTERSECTION software. It is in the current release version and is available for use by thousands of professionals in the USA and elsewhere who have been using the SIDRA INTERSECTION software. It offers substantial extensions to the HCM model capabilities.

    See articles on http://www.sidrasolutions.com/Resources/Articles

    Rahmi Akcelik

  2. 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.