LORIGNAC, France—When mayor Évrard Garnier needed some roadwork done at the edge of his village, nestled in the countryside north of Bordeaux, he called on one of the region’s leading contractors.

Jean-Luc Plé responded with a monumental sculpture: a pair of 6-foot, lacquered polyurethane snails. Rising from the center of a roundabout, one snail climbs a giant grapevine while the other gazes, like Narcissus, at its reflection in a mirror.

Mr....