Los Angeles city and county leaders did their best Monday to eliminate any ambiguity in their quest to warn motorists about the impending traffic “nightmare” over a weekend in mid-July when construction crews plan to shut down several miles of the 405 Freeway for more than two days.

“There’s gridlock on the 405 virtually any time of the day, but particularly during the rush hour, and if you think it’s bad now, let me just make something absolutely clear: On July 16 and 17, it will be an absolute nightmare,” said L.A. Mayor Antonio Villaraigosa, who was one of nearly a dozen speakers at a news conference outside the Skirball Cultural Center in Brentwood.

“And that’s why we’re asking the public to plan ahead,” Villaraigosa said. “To avoid the area, to not go on the 405 or anywhere close during that period of time.”

The 405 will be closed for 53 hours on the weekend of July 16 and 17 -– beginning the evening of July 15 and ending early the morning of July 18 -– mostly for demolition work on the Mulholland Bridge. The northbound 405 will be closed for 10 miles between the 10 and 101 Freeways, and the southbound 405 will be closed for four miles between the 101 Freeway and the Getty Center Drive ramps. The work is part of a larger $1-billion freeway-improvement project that includes constructing a 10-mile northbound carpool lane to complete what officials say is a network between Orange County and the San Fernando Valley.