# 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:

Capacity Calibration |
HCM2010 |
HCM 6 |
|||

Approach |
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 |
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**

**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.

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

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.

The photos are also mislabeled. The blue spreadsheet is for Vistro.

Thank you for the catch! We’ve updated the labels.

Mike,

re the roundabout critical headways and follow-up headways in your table, I don’t find them in HCM 6 Chapter 22 or Chapter 33. The research publication FHWA-SA-15-050 seems to have different headways for some lanes. What is your source?

Thanks

The information is in the HCM, 6th Edition. You will need to do some basic math to get the numbers we published. Equations 22-1 to 22-7 present the capacity equations for the various lane configurations. Equations 22-21,22-22, and 22-23 present the calibration equations that are used in the capacity equations. Through algebra with these equations, the default critical headways and follow-up headways used in the HCM capacity equations can be determined.

Chapter 22 presents the basic equations. Chapter 33 shows the equations in use through examples. So while not explicitly listed, the HCM 6th Edition is the source.