A billboard is a poor channel for inspiring action. When you’re walking down the street and see a billboard, even if the message creates desire to buy the product it’s advertising, actually making the purchase has to wait until later when you may well have forgotten ever seeing the billboard in the first place.

What’s missing from a billboard—and a lot of other communications—is the ability to do what the advertiser hopes you’ll do. Ability is one of three requirements for influencing behavior, according to Stanford professor B.J. Fogg. The other two are a trigger and motivation. But, according to Fogg, ability trumps motivation.

Fogg—who studies behavior design—spoke at this year’s IABC Research Foundation lunch, held Tuesday at the association’s world conference in San Diego.

Fogg’s observation got me thinking immediately about mobile communication. I’m routinely dismayed by the number of people I talk to whose companies aren’t developing strategies for communicating via smartphones and tablets, since more people will go online with these devices than tethered computers by 2014.

Business needs to give the transition careful consideration. What people will look for on their phones when they’re standing on a street corner isn’t likely to be the same as what they want when sitting in front of a 29-inch monitor. How they use smartphones is also likely to be different from how they use tablets, which they won’t carry in holsters wherever they go.

Companies also need to determine what content and services can be made more valuable when it’s mobile, just as Sony’s Walkman increased the value of personal music collections: Listening to your music no longer required you to be home, the fixed location of your hifi equipment.

But mobile can also create ability where it didn’t exist before. A QR code or SMS number on a billboard suddenly makes it a good device for issuing a call to action. (For pedestrians and passengers, anyway—drivers should keep their eyes on the road.)

For more on triggers, ability and motivation, take a look at Fogg’s Behavior Model. It’s as good a place as any to start thinking about your strategy for the shift to mobile online content.