Everytime I hear people complain about rotaries in Massachusetts I have to laugh. Allegations of the state having too many rotaries are absolutely absurd, and assumptions that rotaries are a dumb idea are based on misinformation and a complete lack of understanding of traffic patterns.
Many European countries such as England, France and Spain make ample use of rotaries in managing certain types of intersections because they understand the tremendous value of rotaries to keeping traffic flowing and reducing signal problems and accidents. Despite the cliche image of Clark Griswold taking his family on a European Vacation and being stuck on a London rotary all day, rotaries serve a tremendous good and are far underused and greatly misunderstood in the Commonwealth.
Rotaries work by having traffic enter a circular roadway which has some sort of barrier in the center. The barrier can often be a large landscaped island but can sometimes be as small as a disk in the road. Instead of traffic approaching the rotary needing to rely on its turn to determine how it enters the intersection, all cars enter the rotary the same way. The advantages to this are numerous:
- Cars will only enter the rotary when there is a gap in the traffic.
- Cars will not need to signal, and lack of signaling will not cause accidents.
- Cars will not need to ever anticipate the moves of oncoming traffic to turn left.
- Cars will never pause if unsure.
- Cars will always have to slow down when approaching the rotary.
Similarly, exiting the rotary is benefits in the same way. The safety and timeliness of exiting the rotary is always the same, never relying on running a turning signal or crossing a stream of traffic. Backups which often occur at lights where traffic is blocked by a car waiting to turn left, are avoided and the flow of traffic is continuous as long as exiting roadways are not blocked (and this speaks to good rotary design).
Well sometimes. Rotaries only improve things when they are well designed and well located. Some intersections do not lend themselves to rotaries and are better suited to either ramps or signals. Generally speaking the key to insuring that a rotary is going to work well is having enough space for it, and insuring that exiting traffic is not blocked. As soon as you block and exit, you create the potential for an entire rotary and all incoming traffic to backup.
The major advantage to a rotary over a traffic signal is that traffic is continuously entering and exiting. A traffic light forces left turning traffic to wait (either for a signal or an opening). A traffic signal also distributes traffic inequitably because at a given moment, it may be opening the flow of heavy traffic in one direction (lets say north south) but uselessly giving time to the opposite direction (lets say south north). If the bulk of traffic is going east west and south north, then the light can only service these two busiest directions at different times. A rotary can service all heavy traffic simultaneously.