Maximum Volumes for Roundabouts

By Bryant Ficek, PE, PTOE

Our various initiatives across Spack Enterprise have given us a wide customer base and contacts across the world. We often receive questions or comments from our clients or readers and are happy to provide a quick thought or comment. Here’s a recent one we received:

“Is there a maximum traffic volume above which no roundabout can work efficiently and hence other types of intersections (eg. an interchange) needs to be considered?”

The short answer is no.

Like other types of intersections, you could theoretically keep adding approach lanes and circulating to accommodate more traffic. Luckily, the Minnesota Department of Transportation (MnDOT) developed some guidelines that we like to use for an initial review. As MnDOT states “Table 1 is included as a guide to assist in determining which intersection options should be evaluated based upon combined Average Daily Traffic (ADT) volumes.”

Table 1 – Potential Intersection Control by ADT Volume

MnDOT’s Intersection Control Evaluation, Fall 2007

Keep in mind that these daily volume ranges are by no means an absolute value for a roundabout or the other types of control. Additional factors besides volume are usually a factor in choosing the intersection control method and some factors may extend (or cut short) this volume range.

For instance, according to this chart, roundabouts could be acceptable for daily volumes up to 80,000 vehicles per day. However, the actual volume limit of a roundabout will depend upon the number of lanes for each approach, the number of circulating lanes, amount of left turn or conflicting traffic, and even if one or more approaches is signalized.

To further assist with roundabout evaluation, our neighbors at the Wisconsin Department of Transportation (WisDOT) have a helpful guide. Based on information from the Federal Highway Administration, the WisDOT Facilities Development Manual Table 20.1 indicates the typical daily service volumes (per day) for a 4-legged roundabout as:

• Less than 25,000 for a single-lane roundabout
• 25,000 to 45,000 for a multi-lane roundabout, 2-lane entry
• 45,000 or more for a multi-lane roundabout, 3-lane entry

Between this information from MnDOT and WisDOT, we can quickly narrow down whether a roundabout is an appropriate choice and what type of roundabout is sufficient for the volume. However, while these guidelines will get you started, the ultimate answer will be in regard to the individual characteristics of each intersection.

Want to make it easier? Check out our Roundabout Analysis Spreadsheet. We’ve put the HCM 6^th 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 or our Roundabout Capacity Guidelines article.