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Rotary or roundabout . . . which is it?

A rotary, like this one in Greenfield, is larger and has speed limits of up to 40 miles per hour, according to UMass Amherst experts.Ryan Wicks for the University of Massachusetts Transportation Center

Is there a difference between a rotary and a roundabout? Transportation experts at the University of Massachusetts Amherst have the answer.

In the first video of the University of Massachusetts Transportation Center’s series, “Transportation Take-Away,” center director Michael Knodler compares and contrasts the two circular traffic patterns.

Roundabouts are the more modern version of a rotary, Knodler explained in the video. They’re smaller, slower, and safer, he said. Massachusetts has even been phasing out rotaries in favor of roundabouts, the Globe reported last year.

The easiest way to tell one from the other is its size. Modern roundabouts are much smaller than rotaries.


While rotaries can have speed limits as high as 40 miles per hour, roundabouts permit speeds of just 15 miles per hour.

Roundabouts also have fewer “conflict points” for potential collisions, Knodler said. A typical traffic signal intersection has 32 such points; a roundabout has only eight.

Some rotaries pose additional risks by allowing parking and giving entering traffic the right-of-way.

To muddy the matter further, a traffic circle can refer to either a roundabout or a rotary, although in New England it typically means the latter.

The center’s video was inspired by a recent debate in Amherst over a new roundabout.

And for future traffic engineers who may find themselves in Knodler’s class at UMass Amherst, beware: Students who confuse rotaries and roundabouts won’t soon live it down, he said.

Knodler said he realized the importance of bringing the public up to speed on transportation issues while watching the news on television with his family. After a handful of transportation-related stories, he found himself thinking, “I wish the public knew this.”

The video series seeks to answer commonly asked questions and raise understanding of topical issues such as automated vehicles, marijuana regulations, how speed limits are set, and how cities and towns prepare for New England winters.


“Transportation impacts every person every single day,” he said.

For those who don’t drive, the next video will touch on bike shares, which stirred up some recent controversy in the Pioneer Valley.

Each video will be hosted by an expert affiliated with the center. Knodler said he hopes the videos can narrow the knowledge gap between the public and those who design transportation.

“We set out to have a little fun with it, but also to share some information on questions we routinely get,” Knodler said.

Morgan Hughes can be reached at morgan.hughes@globe.com.