All roundabouts share five characteristics. First, entry to the intersection is controlled by a yield sign, while no such controls exist inside the circular roadway. Second, vehicles inside the circular roadway always have the right-of-way. Third, pedestrians can only cross the legs of the intersection, and they can only do so behind the yield sign. Fourth, parking is not allowed within the circular roadway or at the entries. And finally, all traffic must pass to the right of the central island in a counterclockwise direction.
Despite those limiting factors, roundabouts come in a surprising number of shapes and sizes. The most familiar ones feature a round central island surrounded by a circular roadway and three or more intersecting roads known as legs. While the sizes vary depending on circumstances, design standards suggest a diameter between the 45-foot (13-meter) "mini-roundabouts" sometimes used in small, urban intersections and the 200-foot (60-meter) "rural roundabouts" built to accommodate higher entry speeds. Additionally, this type of roundabout typically has one or two lanes, depending on how many cars need to move through the intersection [source: FHWA, "Roundabouts: An Informational Guide"].
Other designs are a little more complex. Take the "dog bone" roundabout, which looks like someone took the circle and squeezed the middle of it. Similar to the dog bone is the "dumbbell" roundabout, which is actually two roundabouts connected by parallel traffic lanes. Then there's the "hamburger" roundabout, which looks like a traditional circular roundabout, but the main road actually crosses the center island. "Flower" roundabouts also look like traditional roundabouts, but right-hand turns are routed outside the roundabout on segregated "slip lanes." Finally, the "turbo" roundabout features a spiral design that prevents drivers from changing lanes in the circular roadway by requiring them to choose an entrance lane based on where they want to exit [source: National Motorists Association]. The strangest roundabout of all, though, has to be Swindon, England's 7-Circle Magic Roundabout, which you'll just have to see to believe.
Roundabout design isn't just about moving cars, though. Pedestrians, for example, are often provided crosswalks on the legs behind the yield signs, and a feature called a landscaping buffer is used to keep them out of the intersection and direct them toward the proper crossing. Cyclists are expected to ride with traffic or jump up on the sidewalk and cross like a pedestrian, so some roundabouts provide bike ramps that transition from the entrance legs to the sidewalk. And then there are vehicles, like 18-wheelers, that are just too big to negotiate the tight turns in roundabouts. For these vehicles, some roundabouts feature aprons, or a low ring around the central island that's off-limits to cars but open to truck drivers if they need more space [source: FHWA, "Roundabouts: An Informational Guide"].