How Roundabouts Work

While roundabouts may have more fans in Europe, they're actually rooted in the United States. The basic shape goes all the way back to the 1790s, when architect and engineer Pierre L'Enfant proposed a number of circular intersections in Washington, D.C.'s street layout, including the famous Dupont Circle [source: Waddell]. American businessman William Eno revived the form in 1905 for New York City's Columbus Circle, widely considered the country's first circular intersection designed for the automobile era [source: FHWA, "Roundabouts: An Informational Guide"].

Like modern roundabouts, these junctures feature one-way traffic moving around a central hub and connecting roads branching out like spokes on a wagon wheel. But the central circle is much larger, and connecting roads enter at abrupt right angles, requiring drivers to significantly slow or stop before entering the intersection. They're what engineers call traffic circles.

In the 1930s American road departments began building rotaries, the next step in roundabout evolution. Like modern roundabouts, these intersections featured connecting roads that entered at a more gradual angle, allowing traffic to merge into the central circle at a higher rate of speed. Engineers also employed triangular islands, or splitter islands, to separate entering and exiting lanes [source: Waddell]. But there was one big difference: Vehicles within the central circle had to yield to those entering the intersection. Engineers figured as long as they gave cars plenty of room to change lanes between an entry and the next exit, traffic would flow smoothly. Not only did this design lead to huge rotaries, it also caused congestion and high crash rates, causing the design to fall out of favor in the United States by the mid-1950s [source: FHWA, "Roundabouts: An Informational Guide" and Waddell].

Then came the British to the roundabout rescue. In 1966 they experimented with a rule that required vehicles entering a circular intersection to yield to those already in it. Capacity increased by 10 percent, and delays and crashes both decreased by 40 percent. It was a huge success, and the country soon implemented the rule nationwide. The modern roundabout was born [source: Waddell].

Over the next few decades, the British concept spread throughout the world, though some countries proved more skeptical than others. The United States, which had actively removed many of their old rotaries, was particularly resistant: A proposed three-leg roundabout in Ojai, California, would have been the country's first had it not been scrapped due to public outcry in 1988. It would be another two years before the United States got its first modern roundabout, when engineers constructed two in the planned community of Summerlin, Nevada [source: Waddell]. Despite some persistent American skepticism, the concept continued to gain traction, and by 2014 the country boasted 10,341 roundabouts [source: Metcalfe].

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"].

The more roundabouts that are built in the United States, the more popular they become. Why? Because people can see the benefits of a safer, less congested, lower cost, more attractive and more environmentally friendly intersection. A survey conducted in 2001 questioned drivers in three communities before and after roundabouts were built. Those favoring the new intersection jumped from 31 percent before construction to 63 percent after [source: Insurance Institute for Highway Safety].

Roundabouts are safer than traditional intersections for one big reason: Fewer conflict points. Think about how many places traffic can cross paths at a signaled intersection. There are a lot — 32 to be exact. In a roundabout there are only eight such places, greatly reducing the potential for collisions [source: Nevada Department of Transportation, "Roundabout Benefits"]. What's more, their design, with one-way traffic and angled entry lanes, reduces the chance of dangerous T-bone or head-on collisions to zero. Roundabouts also have a slowing effect, forcing drivers to brake as they navigate around the central island and eliminating the impulse to speed up and beat a traffic light [source: Washington State Department of Transportation, "Roundabout Benefits"].

All these features can produce dramatic safety improvements. A 2000 study that looked at 24 newly constructed roundabouts across the United States revealed a 76 percent decrease in injury crashes, a 90 percent decrease in fatal or incapacitating crashes and a 39 percent reduction in overall crashes [source: FHWA, "Roundabouts"].

Pennywise city budgeters will also find a lot to love in roundabouts. While the cost to construct a roundabout is similar to that of a signaled intersection, the cost to maintain the circular intersection is much lower. Traffic lights cost about $5,000 to $10,000 in yearly maintenance and have a lifespan of 15 to 20 years. Compare that to a roundabout, which lasts about 25 years and has none of the hardware, maintenance and electrical costs of signals [sources: WSDOT, "Roundabout Benefits" and NDOT, "Roundabout Benefits"].

Of course, safety and cost savings are great and all, but it's the reduction in traffic congestion that gets the public most excited about roundabouts. With no stop signs or traffic lights to halt traffic, roundabouts promote a continuous flow of vehicles that can really reduce delays — anywhere from 13 to 90 percent depending on the intersection [sources: WSDOT, "Roundabout Benefits" and NDOT, "Roundabout Benefits"]. That's good news for Earth, too: Improved vehicular flow reduces fuel consumption and greenhouse gas emissions by 30 percent or more in some cases [source: NDOT, "Roundabout Benefits"]. And even if you did get delayed in a roundabout, at least you could admire the landscaping on the central island, an aesthetic improvement over the unbroken concrete in a traditional intersection.

We get it: Roundabouts can be a little intimidating when you approach one for the first time. Nobody wants to mess up and cause an accident. Thankfully, cars, trucks, cyclists and pedestrians can all make it through safely by following a few simple rules.

As you approach a roundabout in your car, you'll notice a yellow, diamond-shaped sign with a circle of arrows denoting the roundabout ahead. It will also have suggested speed, usually around 20 to 30 miles per hour (32 to 48 kilometers per hour). Slow to that speed and look for pedestrians in the crosswalk. If the walk is clear, continue to the yield sign, checking to your left for any traffic in the circular roadway. If it's occupied, stop at the dashed yield line; otherwise, you're good to enter the roundabout.

Now you have the right-of-way, so don't stop. Not only will you disrupt the flow of traffic, but other drivers will probably let you know about it with their horn or other, well, gestures. Once you reach your chosen exit, signal to indicate your intention to turn and check again for pedestrians. The only thing that should stop you now is if the crosswalk is occupied.

Two-lane roundabouts require a couple of additional pointers. Upon approaching one of these, you'll also see a sign to help you choose your lane. In a typical four-leg roundabout, you'll want to be in the right lane to turn right or go straight, and in the left lane to go straight, turn left or make a U-turn. Once in the roundabout, don't change lanes or overtake a vehicle or cyclist in the lane beside you. Don't travel next to large commercial trucks, either: They may need to use both lanes to make it around the circle [sources: WSDOT, "How to Drive" and NDOT, "Driving"].

When cycling you should exercise care as well, whether you choose to ride though a roundabout like a car or pull over and walk it like a pedestrian. If you decide to ride, occupy the center of the lane in order to discourage cars from passing you, and don't forget to signal before you exit. If that's a little too much action for you, you're welcome to use the crosswalk; just make sure you get off your bike and walk it like a pedestrian.

Speaking of pedestrians, they also have a role to play in crosswalk safety. When walking through a roundabout, never cross the circular roadway. Instead, cross the legs about one vehicle-length from the circle, preferably at a crosswalk. Even though you have the right-of-way, make sure drivers see you before you step out into the road. If you need to, use the splitter islands for refuge [source: NDOT, "Driving"].