The 146th Street and Allisonville Road roundabout with an overpass, for example, costs an estimated $44 million. The roundabout in Fishers cost $6.8 million.

The “turbo roundabout” at the intersection of highways 25 and 156 in Hollister, Calif., on Tuesday, April 1, 2025. (Nhat V. Meyer/Bay Area News Group) ...