A stretch of Vashon Highway just south of Burton will be closed for a month starting next week so that King County road crews can replace eight aging culverts, county officials announced Friday.

The highway will be closed from March 8 to April 9, the longest closure of the highway in recent history, said Linda Thielke, a spokeswoman for the King County Road Services Division. The repairs are needed, she said, because the current culverts are too small.

During heavy rains, “the water backs up and ponds on the roadway, which can be a traffic hazard,” she said. The drainage system, she added, is “in dire need of repair.”

Road crews will attempt to open up the road by 4 p.m. every afternoon; they’ll also likely have the road open on the weekends. The county will set up electronic signs next week that will let drivers know the road’s status on a daily basis, Thielke said.

The actual closure will be along the highway near the Magnolia neighborhood, between 115th Avenue S.W. and S.W. 242nd Street. The county suggests a detour that would take drivers up Burton Hill, east on S.W. 232nd Street and south on Wax Orchard Road.

The displacement could be considerable. The county’s closest traffic count — at Vashon Highway and S.W. Quartermaster Drive — recorded 5,950 vehicles heading north or southbound on an average weekday.

Thielke, however, said the work is critical to the road’s structural integrity and safety. The eight concrete culverts currently under the road are 12 to 18 inches in diameter; they’ll be replaced by 24-inch culverts.