R Moses' Answer:
This is an important question and something I’ve seen a lot of cities considering lately.
First, roundabouts can even out traffic flow compared to typical signalized intersections which tend to create stop-and-go lineups of cars. But whether they can relieve truly congestion depends on what is causing it, which could be other issues outside of a single intersection. If a roundabout is installed, consideration must be given to the other intersections around it, and light timing must be spaced correctly so as to avoid backups.
To really get a handle on congestion, multi-modal, long range planning is important. If there is congestion at the beach, promoting alternative means of access (walking, biking or transit), parking management, and other options should be part of the mix before spending a huge sum of money on a project that will, at best, benefit only people driving. A plan for roundabouts that are primarily designed to move cars more quickly and send people walking or biking underground will not help you build a strong town.
Safety is another matter that depends on how the roundabout is designed and if it accounts for the various users, especially people who aren’t in a car. (I answered a question specifically about pedestrian safety and roundabouts a couple years ago.) A roundabout designed for high speed is not safe to begin with, and just gets worse when combined with unfamiliar users and multi-mode use.
On the whole, single lane roundabouts (i.e. one circulating lane) are safer than traffic signals, primarily because of their lower speeds. By now, we have a lot of roundabouts installed in this country, and the data shows that single lane roundabouts save lives and reduce injuries.
An experienced design team that can sort through many alternatives and come up with an optimum configuration is important. Where possible, do a pop-up intersection to try it out. This helps not only to refine the design but also to create community buy in. Depending on how much room is available, you might even consider creating an attraction within the roundabout, like a public plaza or fountain.
No matter how well designed or tested, though, don't expect to get public plaudits. Many Americans are still not convinced that roundabouts are more cost-effective or beneficial for road users than other solutions, so the best for which we can often hope is tolerance.
In closing, I’ll leave you with a more overarching question to consider: Why does your city want to decrease congestion? For a tourist community that relies on visitors shopping at local stores and frequenting local restaurants to support its economy, congestion can be a good thing. It means people are passing by more slowly and taking in the amenities that your town has to offer. Overall, the more you can encourage tourists and beachgoers to get out of their cars and into nearby businesses, the more your city will benefit.
Note: R. Moses is not meant to be professional engineering advice nor should be relied upon as such. Consult your own technical professional before proceeding with your own project.
How would you respond to this question? Jump in with your answers in the comments.