LOS ANGELES - Despite being virtually outlawed by the city three years ago, advertising trailers are spreading like a virus across the San Fernando Valley, drawing the ire of officials and the attention of vandals.

Parked on the shoulder of major thoroughfares, the 4-by-6 signs on miniature trailers advertise everything from professional services to fast-food restaurants, although the messages on many of them have been obliterated by vandals in recent days.

"I refer to them as something you wouldn't be able to put in the paper," said Los Angeles City Councilman Dennis Zine, who backed a 2005 ordinance restricting unhitched trailers from being parked on city streets.

"They are a nuisance, an irritant, an eyesore," he said. "We have a quality-of-life issue here."

But entrepreneurs who have created a cottage industry out of the ads say their signs are legal, and they are just trying to make a living.

"We're not looking to hurt or offend anybody," said Bruce Rosensweig, who started the AD-A-Glance trailer-sign company about a year ago.

After receiving 12 phone calls in response to a mini-billboard he placed on Woodlake Avenue in West Hills, Rosen- sweig realized he had stumbled on a lucrative business idea.

Today, he has about 40 ad trailers parked across the San Fernando Valley and other parts of Los Angeles, beckoning customers to buy breakfast at Wendy's or call a toll-free number for credit counseling. He has plans to expand to cities in Arizona and possibly to Las Vegas.

Despite his success, Rosensweig can't sit back and watch his business grow. He's always on alert to move one of his trailers if he gets a call from an angry resident or from the Los Angeles Department of Transportation.

According to the 2005 ordinance, no trailer can be left unhitched on any city street where a sign prohibiting such parking is posted. The problem is that signs haven't yet been erected on many streets.

That has city officials and trailer owners playing a game of cat and mouse - with the businessmen trying to move the billboards before they are cited or towed.

A spokesman for Zine said the councilman has ordered dozens of the no-trailer signs erected, but the work hasn't yet been done. Zine also has asked to meet with City Attorney Rocky Delgadillo in the hope of tightening rules to get trailers off the streets.

Some cities, including neighboring Simi Valley, have a blanket ban on mobile advertising vehicles.

But Zine's spokesman said Delgadillo advised against an outright ban when the 2005 ordinance was being drafted, fearing that it would invite lawsuits asserting the owners' free-speech rights.

Because of the ambiguous rules, Rosensweig has created a complex system to track his dozens of trailers, ensuring that none sits in one place longer than the three days allowed under another city ordinance.

Nevertheless, he gets frequent calls from city workers or an advertiser, asking him to move a trailer because of residents' complaints.

He said he tries to comply, but he occasionally fails to move quickly enough. A couple of times a month, Rosensweig finds himself paying a $55 citation or a couple of hundred dollars to recover a towed trailer from the impound lot.

And he admits that if no one complains, he leaves the trailers where they are - sometimes for weeks at a time.

Several signs parked along Shoup Avenue in Woodland Hills have been there for weeks, for example, despite being painted over by vandals.

Lt. Jody Perez, area commander for the Department of Transportation's Valley enforcement office, said her staff fields about 300 calls a day about abandoned vehicles, many of which are ad trailers.

She said her staff cites or impounds trailers that are violating the law, but that officers' hands are tied if a vehicle is moved even an inch every three days.

"There isn't much more we can do," she said.

The controversy about trailer signs, which led to the ordinance restricting them, erupted about four years ago when roughly 100 curbside billboards began appearing on city streets advertising a security company.

Bruce Boyer, owner of Lone Star Security & Video Inc., said he still has about two dozen trailer signs, advertising his firm, parked along the city's streets.

He said that even though he keeps meticulous records about moving the signs before the 72-hour rule kicks in, city workers tow several a month.

Still, he said, paying to get the signs out of impound - which costs about $200 a pop - is well worth the headache.

"It is the most effective advertising we have ever done, and I have done everything," he said. "Those 24 little vehicles are probably worth $24,000 a month in business."