Compared with stop signs and traffic lights, roundabouts are significantly safer, engineers say. For example, crashes that result in serious injuries or death are reduced by 82 percent versus a two-way stop, and by 78 percent compared with an intersection with traffic lights, according to Jeff Shaw, the intersections program manager for the Federal Highway Administration.

Mr. Retting of the insurance group said that the reduction in injuries and fatalities was “unmatched by anything else we can do in traffic engineering.”

Unlike standard intersections, drivers cannot speed across a street and hit a vehicle in the perpendicular lane; instead, they must slow and merge with others in the circle. Left turns in front of oncoming traffic are eliminated. And because vehicles never come to a complete stop, less fuel is consumed.

And there is even a side benefit: If drivers are not familiar with the area, they can circle endlessly until they figure out their route — think Chevy Chase’s character in “National Lampoon’s European Vacation.”

The federal government is a big supporter of roundabouts. “Our interest is to have their numbers grow,” Mr. Shaw said. “All the states have come around and embraced them. We’re seeing hundreds of new ones every year.”

While federal dollars typically pay for the bulk of the cost of local safety improvements, roundabout construction can be promoted with funding incentives, with some qualifying for 100 percent federal funding, Mr. Shaw said.