Thursday, April 26, 2012

Rotaries vs. Roundabouts

By: Laura Beth Solomon

Many people don't understand the differences between a rotary and a modern day roundabout. If you try to google "rotary" or "roundabout" most sites say the terms are interchangeable. In Massachusetts, this confusion is extremely evident. Rotaries were the old way of designing circular traffic movement. Most people are stuck in the past and now when a modern day roundabout is introduced, they instantly think about a rotary. This blog will help separate a rotary from a roundabout and show just how functional a modern day roundabout is.

A rotary is very large in diameter. Rotaries are typically a hundred to a few hundred feet across. Because the circle is so large, traffic moves very quickly. An important aspect is the tangential entries and exits. Speeds are 30-40 mph or higher because vehicles can drive straight onto the rotary with no little or no deflection. The tangential entries also make it confusing for drivers. When approaching, there is typically a yield sign. Drivers must yield to the circulating traffic, but some don't since their entrance is straight. Once in the rotary, there is a lot of weaving between the two lanes. Rotaries will either have no striping, or dashed lines so cars know they need to weave between the lanes to get where they are going. Typically, rotaries have yield signs at the entry points, but it usually only applies to the inside lane. The outside lane doesn't have to yield and can go quickly to the right and get right off the rotary. Most rotaries were designed in the 1940's or earlier so they can't handle the amount of traffic volume on todays roadways. Most of the time, rotaries are under capacity. When under capacity, accidents occur and vehicles travel at higher speeds. Vehicles can experience delay when trying to get on the rotary. Cars weaving between lanes near the entries and exits creates traffic jams. These traffic jams decrease the capacity of the rotary and increases delay for vehicles.
Rotary in Methuen, MA connecting I-93, Route 110, and Route 111

Google Maps
As you can see from the example rotary, it is very large in diameter and fosters high speeds. Cars coming off the interstate can travel right onto the rotary if there is a gap. During rush hour, the traffic backs up onto the off ramp and blocks the first lane of the highway in both directions.

A modern day roundabout is very different from a rotary. First, a roundabout is much smaller in diameter. In a roundabout, the entering traffic approaches at a smaller angle than that of a rotary. Vehicles enter at an angle closer to 90 degrees. Drivers know they must yield before entering the roundabout. Because the diameter is smaller, and all cars must yield, the speed of traveling vehicles is approximately 25 mph or less.

Roundabout in Brookline, MA
Google Maps
Another reason vehicles travel at slower speeds is that they are deflected. No vehicle can travel straight through the roundabout. All traffic is deflected around the center island and forced to only make right turns. This is much safer for vehicles as well as pedestrians and cyclists trying to cross.

Roundabout Characteristics
Source: NCHRP Report 672

When vehicles approach a roundabout, they choose which lane they need to be in before entering. This prevents cars from weaving between lanes once in the roundabout. The roundabout is striped as a spiral and cars can't change lanes once in the roundabout. Roundabouts were developed in the 1960's and are able to handle a lot more traffic efficiently.

Roundabout in Wyoming

Roundabout in the Netherlands
Roundabouts are prevalent in European countries. The Netherlands has a number of roundabouts that all function extremely well. Roundabouts are slowly coming over to the United States and being implemented.

A roundabout is much better than a traditional four way intersection for a few reasons

  • A roundabout has 8 conflict points for vehicle collisions while a four way intersection has 32
  • Roundabouts reduce user delay
  • Pedestrians and cyclists can cross a roundabout easier because there is a refuge island and they only have to cross one direction of traffic at a time
  • Vehicles are going much slower because they have to yield to circulating traffic
  • Crashes in a roundabout are less severe than a traditional intersection

Roundabout in Nantucket, MA
Google Maps
Roundabouts are very different from rotaries in many ways. Most people are hesitant to accept a modern day roundabout in their community because they are thinking about how terrible rotaries can be. Once the public learns about the strengths of a modern day roundabout, they will be implemented a lot more.
Washington Square Roundabout, Worcester, MA
Google Maps


  1. The only difference between a rotary and a roundabout is your preference. They're interchangeable. Though, there is a movement to differentiate by size. Why? No good reason. In the UK, thy only say roundabout because that's what they call it. We have only ever said rotary (in MA, other parts of the US say traffic circle among other names.)

    Don't make it more complicated that necessary.

  2. As a frequent pedestrian a roundabout, traffic circle or rotary is the worst place to cross. You are never certain where the vehicle in the circle is going to exit, turn signs are never used and the pedestrian has to be alert for every car in the circle. As for the cited benefits 3 of the top four cities with the worst accident rates are in Mass. with rotaries the other is DC which also has traffic circles.

  3. Rotary. This is Boston. Sorry, but the Garden is the Garden, a frappe is a frappe and that circular traffic configuration is a rotary.

  4. we have something our town calls at roundabout but the traffic definitely doesn't flow as you describe and since I have almost been hit by a truck and a car where people never yield coming in and there are always accidents and people can and do change lanes I think my town has misnamed it.

  5. You're _supposed_ to stay in the lane in the roundabout. We know how well Boston drivers would observe that rule. I don't know, a roundabout seems like a small rotary where the lines aren't faded.