The name billboard is derived from the term "billing board," and is a large outdoor advertising space designed to capture the attention of motorists and pedestrians. Typically placed around high traffic areas (both automotive and foot traffic), billboards are meant to be simple, striking, and creative. In fact, the creative department in any advertising agency still prizes the billboard space, as it is a golden opportunity to do groundbreaking, impactful work.
The largest size of billboard available in any campaign is known as the "bulletin," but billboard sizes can vary greatly depending on location. However, the typical size of an advertising billboard is 14 feet high by 48 feet wide.
Billboard advertising can be relatively inexpensive depending on the size and location. However, this expense must be weighed against the kind of message you want to get across. As a general rule of thumb, billboards are a very quick read, and therefore should not contain a lot of information.
You also need to consider the content and tone of the billboard creative, as this may have some negative side effects. In the past, sexually provocative billboards have caused accidents on roads and motorways (the Wonderbra ads were notorious for this), and you do not want the negative publicity of car accidents being directly linked to your ad campaign.
Billboards That Go Beyond
There are regular billboards, and there are 3D billboards that really go above and beyond to attract the attention of the viewer (and the awards juries).
Although there are height and width restrictions for billboards along with distraction rules, advertisers have the ability to really push the boundaries and create boards that are newsworthy.
In the past, billboards have simulated car crashes, used smoke, and special effects, and have even had real people living on them. This type of billboard is generally not much more expensive to produce, as the bulk of the cost comes from renting the space. Creativity like this is worth exploring, as it gives you a greater ROI, and offered earned media credit which can get many thousands more eyes on your messaging.