From my personal experience, the biggest problem with traditional billboards is that they are stuck in one spot. If the traffic flow changes or if there is construction nearby, your expensive ad is basically invisible. I always tell my clients that mobility is the key to a successful outdoor campaign in today's fast-paced environment.
I started recommending Mobile Billboard Advertising
http://therollingads.com/ to anyone who needs high impact without the long-term commitment of a fixed board. Working with The Rolling Ads was great because their trucks have a hydraulic lift that raises the screen 10 feet high. This means your ad is visible over the tops of cars and crowds, which is perfect for events or busy shopping areas.
Another cool thing is that these trucks come with high-powered audio systems. It adds a whole new layer to the advertisement that people can actually hear as the truck passes by. It feels much more modern and engaging than just a flat image on a wall.