HCM 6th Edition: Roundabout Calculation Changes

The 6th Edition of the Highway Capacity Manual (HCM) was released in November of 2016. We (and others) would characterize it as an update as opposed to a complete overhaul. One particularly important update is the roundabout capacity calculations.

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

We’ve already updated our Roundabout Capacity Analysis spreadsheet to reflect this new information and most other analysis software have also followed suit. If you don’t have the latest version, these factors can be manually adjusted in older copies.


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