Extensive development of the road network did not start until after World War II. By the 1980s, however, roads were the most important part of the transportation system. Before the war the few existing roads had been intended primarily as feeders to the railroad system, which had been built largely with foreign funds that needed to be repaid. Profit from rail transportation was vital, and the construction of competing roads was deemed uneconomic. From the mid-1950s to the mid-1960s, however, substantial United States aid was provided, along with technical assistance, to develop a national highway system that by 1965 totaled almost 9,500 kilometers. Thereafter, assistance for highway development came mainly from the World Bank, although in the late 1960s United States military forces also furnished substantial funds for road construction.

In the 1980s, the primary road system consisted of a net of national highways that started at Bangkok and extended in all directions to the country's frontiers. They totaled about 20,000 kilometers, of which well over 90 percent were paved. Provincial roads totaling over 24,000 kilometers formed a secondary system that tied provincial towns and population centers to the national roads. About two-fifths were unimproved and often impassable during rainy weather. In addition to the main and provincial roads, there were tertiary roads--consisting of village roads, footpaths, tracks, and the like--variously estimated at from 40,000 to 60,000 kilometers. These roads and trails were important because they represented in many cases the only link between a village or hamlet and the provincial system or possibly a railroad stop or inland waterway point. Several thousand kilometers of tertiary roads had been improved, but in general they were poorly maintained. Their administration was spread over a number of government agencies, in contrast to national and provincial roads, which were administered by the Department of Highways in the Ministry of Communications.

In the early 1980s, no restrictions existed on the importation of motor vehicles, although taxes and duties on imported vehicles were higher as a measure to protect the domestic automobile assembly industry. Under guidelines set in 1986, local automobile assembly plants were required to use at least 54 percent domestic parts. Motor vehicles registered in 1984 included 688,000 automobiles, 600,000 commercial vehicles, and nearly 2 million motorcycles. In the 1980s, about a third of all vehicles registered were in the Bangkok metropolitan area, but this included almost two-thirds of the automobiles. The relatively massive concentration of trucks, buses, and automobiles in the capital area regularly created enormous traffic jams. Construction of an elevated expressway was under way, the first part of which had been completed by the early 1980s.

Data as of September 1987