In general, the most effective forms of media are visual ones. Television is very effective, but also very expensive, and each ad slot must be bought individually.
Radio is fairly cheap but the target audience is narrow and limited, so that often times, multiple ad slots must be purchased.
Billboards are thought to be one of the most cost effective means of advertising, as a flat fee will present your ad 24 hours a day, 7 days a week to everyone who drives by it. The cost to produce a billboard is relatively low, with no actors or studio costs to consider, and it can be left up for months at a time. The target audience for a billboard is everyone who drives a car or rides in one, which is automatically almost everyone in the country. Placement of the billboard, of course, is extremely important in determining its effectiveness.