Las Vegas -- As in most major cities, the taxis in Las Vegas are rolling billboards, with signs on the doors and huge lighted versions on the roof. And why not? Taxi fleet owners are looking to maximize their investment in the vehicles. Part of that is keeping them on the road as much as possible (compared to the five percent of the time the average private car is operating) and another is renting out mobile space for advertisers. Other forms of this include advertising on buses and the traveling billboards pulled by trucks around American roadways.