Roundabouts are popping up everywhere. As transportation professionals, we love them! The reduction in conflict points and severity of incidents make roundabouts one of the most beneficial tools we have in our toolbox. But why aren’t roundabouts at every intersection if they provide such a benefit over a traditional signalized intersection?
The answer to that question could be several different factors. It could be the Average Daily Traffic (ADT) or the space available with the surrounding land uses. A roundabout may not be the best tool for the job on a 6-lane road with 90,000 ADT or in a small urban area with buildings on every corner.
Other factors include time and money. Once a roundabout is selected as the best option for an intersection, the finances must be in place for the city, county, or state. Not only that, but roundabouts take time. Depending on the magnitude of the project, pre-construction planning could take a handful of years. From purchasing right-of-way to coordinating utility relocations and environmental impacts, time can seem to stand still.
Time can also play a key role in the design and construction of roundabouts. Building a roundabout in a new location with no conflicts of existing conditions can be relatively straightforward but converting an existing intersection layout to a roundabout can present unique challenges in constructability and traffic control plans.
A recent roundabout project we have worked on with the Alabama Department of Transportation (ALDOT) is the Hayden Roundabout series in Hayden, Alabama. For this project, constructability and traffic control were a primary focus. For the eastern roundabout, the elevation difference in the existing intersection provided a unique challenge to design around. Space was at a premium surrounding the intersection. So early on in project concept development, we looked at options for construction phasing and traffic control, including phasing under traffic or building an on-site detour alongside the finished roadway or through the adjacent park and ride.
Ultimately, what was selected was a partial closure and off-site detour with an incentive/disincentive package for the closure, meaning that the contractor would be incentivized to re-open the closed road segment as quickly as possible. Even with the incentive package, construction still takes a fair amount of time to work alongside traffic continuing to utilize the roadways. Rome was not built in a day, and roundabouts certainly can’t be either!