A map showing, as its primary feature, highways, byways, and other roads, especially covering an area larger than a city (maps of roads within a city are generally referred to as street maps).
The first road maps in the United States were published in the late 19th century for bicyclists, both by groups such as the League of American Wheelmen and commercial printing companies such as Rand McNally. When the automobile appeared, it was a fairly simple matter for the cartographers to overprint the bicycle maps with another color to indicate roads that were suitable for cars.
However, there was one major problem with attempting to use these early road maps: most roads weren't marked in any way. To successfully navigate, early motorists had to use publications such as the American Automobile Association's "Official Automobile Blue Book" series, a collection of books which gave step-by-step directions, in text form, for getting from one town to another, or Rand McNally's "Photo-Auto Guide" series, which supplemented the text directions with actual photographs of each turn in a given route.
A slightly less bulky solution was the strip map, which was an extremely detailed map of one specific route between two specific cities (the forerunner of the AAA's Triptik system).
Finally, movements began to spread to mark the various roads between cities, some led by local roadside restaurants, inns, and other businesses; some led by automobile products manufacturers, most notably the B.F. Goodrich tire company, with its "Goodrich Guide Post" signs; and one major effort led by Rand McNally itself creating a system of "blazed trails," each with different color markings. The marked roads finally made it easy to produce maps showing all the roads in a given area, although with several competing marking systems, Rand McNally's maps only showed roads they'd marked, Goodrich's only showed roads they'd marked, and so on. The situation was eventually cleared up by the various state governments, which began to assign numbers to their officially maintained highways in the late 1910s; the Federal government began assigning its own numbers to a system of interstate highways in the mid-1920s.
Eventually, the principal source for road maps came to be gas stations and the oil companies that supplied their products, which gave away maps for free. According to legend, the first oil company road map giveaway was in 1913, when the Gulf Oil Company promoted its first gas station in Pittsburgh by mailing a map of Allegheny County to every registered automobile owner in the county. The promotion was successful, and so in 1914, Gulf offered maps of Pennsylvania and other nearby states both through the mail and at the station. Following World War I, other oil companies began to follow suit. Most of these maps not only promoted the company and its stations, they also promoted the idea of driving, with many having beautifully illustrated covers that were true works of art.
The free gas station road map lasted as an ubiquitous concept in the United States until 1973, and the first oil crisis. Suddenly, the oil companies found profits slipping as the price of gas skyrocketed and began to cut services to economize. Suddenly, instead of being handed a free map by a gas station attendant after he filled the gas tank and wiped the windshield, motorists were going inside the station and paying a dollar or more for a map after pumping their own gas. By the 1980s, most oil companies eliminated even the expense of having special maps produced for them, instead selling maps labeled for Rand McNally or the other cartographic company that had produced them. Road maps also came to be available at bookstores and specialty travel stores, next to the travel books.
Reference (and home to lots of pretty pictures): "Hitting the Road: The Art of the American Road Map," by Douglas A. Yorke Jr., John Margolies, and Eric Baker