By Bryant Ficek, PE, PTOE
At a recent Traffic Corner Tuesday webinar, Are Roundabouts a Silver Bullet to Traffic Issues?, we discussed roundabouts including pros and cons, situations that more inclined toward their use, and characteristics that may lend itself toward signal or other type of control. Through that discussion, a key question arose about the capacity of a roundabout. More specifically, are there guidelines that could be consulted without the need for using analysis software?
The answer is yes. The National Cooperative Highway Research Program (NCHRP because the full name is a mouthful) Report 672: Roundabouts An Informational Guide provides a handy graph examining the capacity of roundabouts with up to two approach and/or circulating lanes. The Highway Capacity Manual (HCM) 6th Edition then updated those formulas. Below is a new graph showing the various capacity levels based on the hourly approach and circulating volume.
With the peak hour turning movement volumes, you can easily check the capacity of each approach leg in your potential roundabout.
Want to make it easier? Check out our Roundabout Analysis Spreadsheet. We’ve put the HCM 6th Edition formulas into an easy-to-use spreadsheet that provides the delays, level of service, volume-to-capacity ratios, and vehicle queues for a three- or four-legged intersection (more than four legs could have each approach checked individually with the formulas, but would not be displayed or automatically combined into an overall delay and LOS for an intersection).
Don’t forget to check out our webinar Are Roundabouts a Silver Bullet to Traffic Issues? for more in roundabout information.