moving multi-image sign. This is compared to a 19% retention rate for static signs. Unlike a typical billboard, mobile billboards are able to go directly to their target audience. They can be placed wherever there is heavy foot traffic due to an event – including convention centers, train stations, airports and sports arenas. They can repeat routes, ensuring that an advertiser's message is not only noticed, but that information is retained through repetition. Multi-purpose billboards Billboards may be multi-purpose. An advertising sign can integrate its main purpose with telecommunications antenna or public lighting support. Usually the structure has a steel pole with a coupling flange on the above-fitted advertising billboard structure that can contain telecommunications antennas. The lighting, wiring, and any antennas are placed inside the structure. Other types of billboards Common along highways are free-standing two-sided as well as three-sided billboards. Other types of billboards include the billboard bicycle attached to the back of a bicycle or the mobile billboard, a special advertising trailer to hoist big banners. Mechanical billboards display three different messages, with three advertisements attached to a conveyor inside the billboard. There are also three-dimensional billboards, such as the ones at Piccadilly Circus, London. Traditional billboards consist of a large format advertisement printed on a resistant material such as vinyl; this is placed on a large metal structure, which makes it one of the media with the most visibility and high impact within the advertising field. Placement of billboards Some of the most prominent billboards are alongside highways; since passing drivers typically have little to occupy their attention, the impact of the billboard is greater. Billboards are often drivers' primary method of finding lodging, food, and fuel on unfamiliar highways. There were approximately 450,000 billboards on US highways in 1991. Somewhere between 5,000 and 15,000 are erected each year. Current numbers are put at 368,263, according to the OAAA (outdoor Advertising Association of America). In Europe billboards are a major component and source of income in urban street furniture concepts. An interesting use of billboards unique to highways was the Burma-Shave advertisements between 1925 and 1963, which had 4- or 5-part messages on multiple signs, keeping the reader hooked by the promise of a punchline at the end. This example is in the National Museum of American History at the Smithsonian Institution: Shaving brushes You'll soon see 'em On a shelf In some museum Burma-ShaveThese sort