I often field calls from business owners who are looking to advertise their product or service on a billboard for the first time and at some point in the conversation I usually hear this question: “How do I know it is going to work?”

The question might not be worded exactly like that, but essentially the person is asking me to provide a realistic answer as to how the billboard (or any kind of outdoor advertisement) will deliver on return on investment (ROI). Thats a tough question to answer because you can’t necessarily see tangible conversions (sales) as a clear result of outdoor advertising like you would with say, a PPC campaign or direct-mail coupons. With out-of-home media, variables like creative copy, color, size, placement and campaign length all contribute to potential effectiveness. Granted, the outdoor advertising industry is making strides in providing sounder metrics like the Eyes On measurement system, but a magic pill that gauges the effectiveness of each outdoor advertisement does not exist.

So while I can’t tell you that 85% of all your new business is directly tied to those four billboards you are running throughout the city for three months — I can tell you how to take steps to ensure that your investment in outdoor advertising will directly lead to a point of potential conversion. We can bring the consumer one step closer to a conversion by always utilizing a call-to-action in the messaging.

First, what is a call-to-action? Simply, it is something that encourages a consumer to act. For restaurants using interstate billboards, the call-to-action might be identifying the proper exit and direction to turn at the exit ramp. “Exit 47, turn left” is an example. The ad has informed the consumer where to go in order to complete the action.

The most common call-to-actions used in outdoor advertisements are 1-800 phone numbers and web url addresses. If the consumer calls the 800 number or goes home and looks up the website on the computer, the billboard has accomplished moving the prospect to the next layer of the sale. Now the prospect is either receiving information directly from someone at the company or they are a few clicks away from an actual conversion.

Lets go back to one of the variables — design — and make sure that we are incorporating the call-to-action correctly in the advertisement. Designers need to be mindful of font size, color contrast and placement of the call-to-action on the ad. If the billboard is on the left-hand side of the road, the 800# or the web address needs to be set on the far right of the ad so that it is closer to the road and oncoming traffic. The font size better be large enough to be seen from a distance — especially when consumers are moving at 60 mph and only have about 6-8 seconds to absorb the message. The contrast of the design needs to “pop” too. If the background of the billboard is white and the font with the call-to-action is light blue, I’m probably going to have trouble viewing the copy the first or even second time I pass by the ad. (For more on contrast, check out the color chart featured in our contrast blog.)

While a web address and the phone number of the business might be the most traditional methods of a call-to-action, brands and advertisers that are digitally savvy are using new ways to drive conversions. One current trend in outdoor advertising is to slap a QR (quick response) code somewhere on the creative copy, or even use a giant QR code as the entire ad itself. Consumers with smartphones can scan the barcode, where they are then usually directed to a website that includes videos, coupons / deals or other items that convince the person to take additional steps (more action!).

In addition to QR codes, advertisers are utilizing SMS (short message service) or text messaging to elevate the effectiveness of their campaigns with the ever-growing mobile savvy community.  You often see this approach in television advertising as well as out-of-home campaigns. With verbiage similar to; Text “donate” to 12345, advertisers are using mobile text messaging an instant call to action.

If you noticed, earlier I mentioned I can’t always tell you that 85% of your new business is because of your outdoor campaign. But, sometimes I can. You can make the effectiveness of your outdoor campaign measurable by using unique qr codes, text to numbers, and web addresses for your outdoor campaign. Using a separate number or address to direct users will allow you to track the actions you receive from your out-of-home campaigns independent from your print, radio and television campaigns.

For an even more personal (albeit strictly digital) approach, some outdoor advertisements are using a Twitter handle or Facebook page as the call-to-action. The convergence and integration of social media and out-of-home media was one of the hottest marketing strategies in 2010 and should continue to evolve this year with brands using different combinations of call-to-actions. One thing is clear though — a call-to-action is absolutely necessary in an outdoor advertisement and one of the few ways you can actually ensure that the investment is working.