By Bryant Ficek, PE, PTOE
Through our Traffic Corner Tuesday (TCT) webinars, we’ve been able to have great conversations on various topics from the new Highway Capacity Manual to crash analysis to pedestrian crossings. It’s been a great way to connect with professionals around the world.
One of TCT topics covered roundabouts and the basics we considered in regard to an intersection analysis. A couple days later, Debbie Albert, City Traffic Engineer in the City of Glendale Arizona, followed up with us with the following:
“We are considering installing two roundabouts near a football stadium and after watching your presentation, I would like to take another look at this option. You mentioned in the presentation that there are some challenges with event traffic. Do you know of any examples of where there are roundabouts near large event generators, or is the advice based on simulation/technical analysis?”
We mentioned event traffic as a potential red flag that should have a second look. The main challenge with event traffic is the potential change in balance between approaches. If 80-90 percent of the traffic would be directed to one approach, a roundabout may not be the best option.
Most of our experience is in regard to high school events. Generally, the traffic levels with those games did/does not approach levels that tested the capacity of the roundabout or caused such an imbalance in traffic as to alter the drivers’ expectations of operations. Outside of our direct experience, the City of Manhattan Kansas installed at roundabout at Kimball Avenue/Grand Mere Parkway on a major route to the Kansas State stadium. I wasn’t able to find the original study, but, anecdotally, this roundabout is described this way (you might be able to find more with an internet search):
“…designed and built on a bypass highway on the edge of the city in an area that was becoming heavily residential. It also served as a main route to the football stadium used by thousands of football fans on home game days. Prior to construction there were all sorts of opposition and predictions of ‘disaster’. These can be summarized as follows (The Manhattan Mercury, various dates):
- football traffic would back up 20 miles (32 km);
- recreation vehicles would not be able to get through the roundabout;
- out of town drivers won’t know how to drive through it;
- there would be numerous crashes;
- football fans would be upset and not support the team; and
- new students would not be able to drive through it safely.
“After this modern roundabout (Grand Mere/Kimball) was opened, traffic flowed through it very smoothly. Cars, large trucks, buses and recreation vehicles (RVs) were all observed passing through the roundabout with no problems. On football game days traffic flowed steadily and there never was a queue of more than three or four cars. Subjectively, it could be stated that this roundabout operated quite well. Much of the vocal opposition (to roundabouts) was basically silenced, perhaps disappointed that this roundabout was not the ‘disaster’ they had predicted.“
Our advice is to make sure your analysis of the intersection accounts for both the typical weekday peaks and the pre- and post-event peaks. Using the HCM formulas is a quick conservative way to analyze the volumes that are either based on counts obtained or forecasted. That will provide the formulaic capacity answer. Then a quick review of the traffic balance would provide a qualitative assessment outside of the capacity answer. Although, it should be noted, that there may be options for temporary improvements during the pre- and post-event periods that would avoid the issue with imbalanced approach traffic (close one or more approaches, close the circulating portion of one or more legs, etc.).
Do you have something to add to our advice?