Roundabout Analysis

[RAWeber]

I'm figuring the performance of a two-lane urban roundabout using the FHWA method provided in Roundabouts: An Informational Guide, and I want to make sure that I am doing it right.

I'm taking the chart in Exhibit 4-4, Approach capacity of a double-lane roundabout, entering at the Circulatory flow, straight up to the line and then across to the Maximum entry flow to find my capacity. With the capacity I'm using the delay equation (4-7) and then taking an average of the delays weighted to volume to find the total roundabout delay.

I haven't yet figured out how to translate this into LOS yet, but I'm thinking either the AWSC or Signalized LOS breakdown.

So, what does everybody think?

Robert Weber, EI
Maurer-Stutz, Inc

http://www.maurerstutzinc.com

"I am satisfied that your answer is correct. I am not, however, satisfied that your answer is right."

 

[ACtrafficengr]

If I recall correctly, the FHWA guide procedure is more of a planning level analysis. If you are doing a design analysis, I suggest getting one of the RODEL, ARCADY, or aaSIDRA software packages. Which one is better? Depends on who you ask. ARCADY and RODEL are based on the British empirical method. I don't know if it has been calibrated for US drivers yet. aaSIDRA is based on the Australian gap acceptance model.

One neat thing about RODEL is it has a design feature which takes the approach volume, and calculates different combinations of flare width and taper length that give you the optimum performance. You can then pick the combination that fits your site. Remember that a long taper and small flare will be safer than a short wide taper, for the same delay.