In the world of outdoor advertising, Portland is viewed as hostile territory. Yet some clever billboard owners have found a loophole in the city's law big enough to, well, drive a truck through.

Two mobile messengers, commonly called ad-trucks, rolled around the Rose City last week, each carrying 10-by-22-foot billboards on their backs. One--owned by New York-based Street Blimps--flogged Toyota's youth-oriented Scion, with scheduled stops near Portland State University, the University of Portland and Waterfront Park.

The other, operated by Do It Outdoors, a Pennsylvania company, promoted 7-Eleven's new Vcom financial kiosks in Southeast and downtown. DIO officials say the signs on each truck cost clients approximately $5,000 a week, and their 37 trucks are all booked.

"I came from Minneapolis, advertising Dex phonebooks," says DIO driver Dan Bankhead, who spent 10 hours a day behind the wheel in Portland.

Bankhead left Portland in his dust Friday, but others are sure to follow, as mobile billboards are gaining ground across the country. Here in town, they're slipping through the legal equivalent of a yellow light.

The sign code adopted by the City Council in 1996 bans new billboards in excess of 200 square feet, yet the sign area aboard both trucks measures 220 square feet.

Maybe these are portable signs, which the city defines as "not attached to a structure or the ground." Even here there's a problem: Movable signs can measure only 12 square feet in area (and temporary portable signs can be only 4 square feet). But those regulations are meant to govern small signage such as banners and A-boards outside restaurants.

Then again, city code also bans "signs placed on...a motor vehicle or trailer parked with the primary purpose of providing signs not otherwise allowed by the code." This seems to imply that ad trucks might be legal when moving.

Confused yet? You're not alone. "They fall between the cracks," says City Commissioner Randy Leonard, whose office works with the Bureau of Development Services to regulate signs. "We don't regulate these."

So, at least for now, ad-trucks have a green light on our city streets.