By Lemor Abrams

ROSEVILLE (CBS13) — Roseville city officials say they had to convene a special team to investigate the special event warned about in mystery signs that appeared alongside roads.

The signs were promoting the Aftershock Festival at Gibson Ranch. The problem is, promoters say they weren’t told they needed special sign permits to display them.

The bright orange signs popped up all over Roseville, Rocklin and parts of Highway 65. Only people didn’t know what event it was for—not even the city of Roseville.

“We were curious too,” said public works director Rhon Herndon. “We wanted to find out what special event this is. And we also have a requirement for an encroachment permit required when anyone places something within the public right of way.”

He says the concert promoter posted signs without telling anyone, violating city code and creating what he called a public safety issue.

Clay Busch says festival officials obtained the proper permits and above all, remain focused on giving fans enough routes to avoid a traffic nightmare.

“Our traffic department were very proactive leading into the event after the City of Trees which was the first event on the property..seeing what happened with the traffic issues,” he said.

With a lineup that includes Marilyn Manson and Slipknot, promoters expect tens of thousands to pack Gibson Ranch.

Here’s some advice from the California Highway Patrol for what to avoid if you’re not going:

Motorists are encouraged to use alternate routes during the days of the event. The primary roadways that may be affected by this event are: Elverta Road, Watt Ave. (north Sacramento County), Elkhorn Blvd., 16th Street (Elverta), Baseline Road, 28th Street (Elverta), and Dyer Road.