I tend to use "rotary" to refer to any road crossing that sends all motorists around a center marker (can be a park or a monument) irrespective of whether the traffic engineers will call it a rotary or a roundabout or a traffic circle or the Place de la Concorde.

You want the clarity, read this.  "The free-for-all improvisation that was the hallmark of rotary driving was all gone. In its place was a forced choreography. In other words: rules."

Paralysis by analysis, I say.
The lack of organization on a rotary was both its beauty — cars could move quickly through them if traffic was light, barely touching the brakes — and its chief problem, especially as traffic volumes swelled over the decades. And like Wiffle Ball and space-savers, everyone seemed to have their own set of rules about how it was supposed to work, along with certainty that everyone else was doing it wrong.

“The classic rotary is somewhat of an unorganized free-for-all,” said Jonathan Gulliver, the state’s acting highway administrator. “This opportunity to upgrade them with the principles of a roundabout will make them smoother and less likely to result in a collision.”

The key is doing away with that pseudo dance maneuver that rotaries require, in which cars kind of trade places as they get on and off.

“A rotary is all about weaving,” Boudreau said, “and the reason they are falling out of favor is that lack of definition. By bringing in roundabout principles, we get cars organized and put them in the correct lane. You should not have to change lanes anywhere in a roundabout.”

Rotaries have a high crash incidence, though it is mostly sideswiping and rear-ending, and are nowhere near as dangerous as traditional lighted intersections, which have the potential for head-on and T-bone collisions, said Andy Paul, a state highway design engineer.

Roundabouts are safer still, largely because they involve a “deflection” for entering cars, a bit of reconstruction that slows drivers down and forces them into lanes that correspond with the exit they’ll be taking.
That is, provided each driver knows for sure which exit he is going to take from the roundabout. The design flaw, though, is those multiple lanes. One lane all the way around, no dancing, no lane switching, no tractor-trailers hogging both lanes.

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