People have been slapping ads on vehicles in one form or another since the invention of the wheel, and yet the search for new ways to adorn cars and trucks with messages seems never to stop.

Here's the latest: suspending signs from the back of cars using a system similar to the bicycle racks commonly seen on the rear of automobiles.

In fact, that’s how the idea came about.

“I saw a bike rack on the back of a car, and I thought, ‘Man that’d be great to attach a sign to,’” says Greg Pilon, president of Ads 4 Tow, the San Diego company that came up with this new system.

The advantage, notes Pilon, is that the ads can be put on, switched and taken off in a jiffy, and that separates them from most car signage.

The system doesn’t obstruct a car’s taillights or license plate when in use, making them legal on the road. There’s also a flip-top version of each that can be used to double the size of the sign when the car is stationary.

“There’s nothing out there that’s great that's not permanent or doesn't involve wraps,” says Pilon. “People want something they can take on and off.”

Right now the signage comes in three different sizes: 24-by-36 inches for smaller cars, 24-by-48 inches for minivans and small SUVs, and 30-by-60-inches for full-sized trucks and SUVs.

The main restriction is that the system can’t use signs that include any sort of lights, which would violate local traffic codes. But Pilon says that can be remedied to some degree by using reflective vinyl.

The ad racks are aimed at small business owners, such as real estate agents, who can buy them for between $350 and $775.