One of the drawbacks to a traffic grid system are the number of intersections it creates. Drivers may encounter two-way stops, four-way stops or traffic lights at any of those intersections. One solution designed to keep heavier volumes of traffic moving through popular intersections is called a traffic circle, or a roundabout in England and Europe. A traffic circle allows drivers to merge into a constant flow of traffic around a large circular island. Street signs around the traffic circle direct drivers towards outer exit lanes.
At least, this is how a traffic circle is designed to work. The concept of traffic circles began during the 1870s in England as a way to keep horses and buggies moving through popular and heavily congested intersections. Drivers were supposed to enter the traffic circle or roundabout and immediately head towards the innermost lane. When the traffic cleared, the driver would quickly merge into the outermost lane and onto the appropriate street. In practice, this often proved to be the catalyst for numerous collisions. During the heaviest traffic periods, some drivers found themselves hopelessly trapped in the innermost traffic circle, unable to merge into the flow of traffic.
As problematic as traffic circles may have been, several cities in the United States eventually adopted them as a reasonable alternative to clog-prone traffic grids. A number of smaller cities in the New England area have a traffic circle instead of a town square, with a county courthouse or community park in the center island. New York City and Washington, DC also feature traffic circles inspired by British roundabouts.
Driving through a modern traffic circle can be a challenge, but refinements to the original design have made it much easier. When approaching a traffic circle, a driver should know both the street name and the official road number of his or her desired exit. The exit signs on a traffic circle may or may not mention local street names. As the driver approaches the circle itself, there is usually a merge lane with a sign to yield. Traffic circles generally move counterclockwise in one direction, so the driver should look left for an exiting driver or a gap in the traffic flow.
Once inside a traffic circle, a driver should move to the innermost circle whenever possible in order to avoid oncoming and exiting traffic. This rule is most important whenever the driver's exit is on the far side of the circle. If the driver only needs to take the next immediate exit, then he or she can drive on the outermost ring and exit promptly. As the driver approaches his or her exit, he or she should merge to the outermost traffic lane and signal for an exit. Signaling allows a driver on the oncoming merge lane to know it is safe to enter.
If a driver misses the exit entirely, then he or she must rejoin the traffic flow and drive around the circle again. Stopping in the middle of a traffic circle is rarely a good idea, and there may be drivers pulling out of parking spaces on the innermost part of the circle.
There is another traffic controller called a traffic circle, but it does not operate like a roundabout. This form of traffic circle is often placed in neighborhoods plagued by speeding drivers and unplanned cut-through traffic. In order to discourage both practices, a large circular concrete island is placed in the center of residential intersections. Traffic can still maneuver around this barrier, but only at reduced speeds. The thought of negotiating these traffic circles every day also becomes a deterrent for drivers using the residential streets as cut-throughs or alternatives to major commuter routes.