Energy.Accelerating from a dead stop is the least efficient thing a car's engine can do. By reducing stop-start queuing—and eliminating it at "off-peak times," like the moments at 2 a.m. when you're idling at a red-light at an near-empty intersection—roundabouts not only waste less time than traditional intersections but also less energy, as various studies have confirmed.
Public space. The left-turn lanes mentioned above not only waste time, they waste space. They're merely a temporary parking lot for vehicles that could otherwise be moving. By removing the need for these lanes in every direction, roundabouts can consume less asphalt. (Having to cross fewer lanes is also safer for pedestrians.) Rather than serving as shrines to the paving industry, the centers of intersections can contribute to the overall aesthetic improvement of a neighborhood, while the slower approach and travel speeds (which also mean less noise) are a boon to any sort of street or neighborhood life outside the car.
There are few silver bullets when it comes to traffic, and roundabouts will not work everywhere. (Some intersections are already too busy to consider switching to the roundabout model.) Like anything, they can be poorly designed: You don't want them to look as if someone simply traced "a circle around a coffee can" on a piece of paper, as one engineer has put it. Bad driving behavior can cause them to "lock up" (just as one driver "blocking the box" can freeze a four-way intersection). Yes, there will perhaps have to be some minor educational outreach—one Indiana town is weighing spending $24,000 to do just that—but a larger question here is whether people who cannot manage to merge at low speed into a counter-clockwise circle and, yes, perhaps even change lanes in that circle, before finding the correct exit should actually be holding licenses that enable them to operate heavy machinery in the first place.
Luckily, though, there are signs that our national roundabout aversion may not last forever. In places like Clearwater Beach, Fla., residents have actually petitioned for roundabouts to be installed, even holding a party on opening day. When's the last time that happened for a traffic signal?