Thai highway network

The Thai highway network follows the left-hand traffic rule of the road. The network is the twin responsibility of the Department of Highways (DOH, กรมทางหลวง, Krom Thang Luang), and the Department of Rural Roads (DORR, กรมทางหลวงชนบท, Krom Thang Luang Chonnabot), under the oversight of the Transportation ministry of Thailand. Public highways (ทางหลวง, thang luang) are also called public roads (ถนนหลวง, thanon luang), especially when part of urban streets. The network spans over 70,000 kilometers across all regions of Thailand. Most are single carriageways. Dual carriageways have frequent u-turn lanes and intersections slowing down traffic. Coupled with the increase in the number of vehicles and the demand for a limited-access motorway, the Thai Government issued a Cabinet resolution in 1997 detailing the motorway construction master plan. Some upgraded sections of highway are being turned into a "motorway", while other motorways are not being built from highway sections.

Types of highways
The 1992 Highway Act (พระราชบัญญัติทางหลวง พ.ศ. 2535), revised as the 2006 Highway Act (พระราชบัญญัติทางหลวง (ฉบับที่ 2) พ.ศ. 2549), defines the following five highway types:

A special highway (ทางหลวงพิเศษ) or motorway is a high capacity highway designed for high speed traffic, for which the Department of Highways carries out construction, expansion, upkeep and repairs, and is registered as such. Motorway entrances and exits have controlled access, and controlled by the DOH. Registration of motorways is overseen by the Director General of the DOH.

A national highway (ทางหลวงแผ่นดิน) is a primary highway, part of the network connecting regions, provinces, districts, and other important destinations, for which the DOH carries out construction, expansion, upkeep and repairs. Registration of national highways is overseen by the Director General of the DOH.

A rural highway (ทางหลวงชนบท) or rural road is a highway for which the Department of Rural Roads carries out construction, expansion, upkeep and repairs. Registration of rural highways is overseen by the Director General of the DORR.

A local highway (ทางหลวงท้องถิ่น) or local route is a highway for which the local administrative organization carries out construction, expansion, upkeep and repairs. Registration of rural highways is overseen by the provincial governor.

A concession highway (ทางหลวงสัมปทาน) is a highway for which a legal government concession has been granted. Registration of concession highways is overseen by the Director General of the DOH.

Highway numbering
The first digit of a highway number indicates the region of Thailand it serves, with the number of digits indicating the highway classification. These regions are:
 * 1) Northern Thailand.
 * 2) Northeastern Thailand.
 * 3) Central and eastern, including the upper south.
 * 4) Southern Thailand, except the upper south.

A single digit indicates one of four highways connecting Bangkok to outlying regions:
 * Route 1 (Phahonyothin Road) to northern Thailand
 * Route 2 (Mittraphap Road) to northeastern Thailand
 * Route 3 (Sukhumvit Road) to eastern Thailand
 * Route 4 (Phet Kasem Road) to southern Thailand

Two digits indicate a principal highway within a region, such as Route 22 in the northeast between Udon Thani and Nakhon Phanom.

Three digits indicate a regional secondary highway, such as northeastern Route 202 between Chaiyaphum and Khemarat, and central Route 314 between Bang Pakong and Cha Choeng Sao.

Four digits indicate an intra-province highway connecting a provincial capital to its districts, or between important sites, such as northern Route 1001 between the Route 11 intersection and Amphoe Phrao, and southern Route 4006 between the Route 4 intersection (Ratchakrut) and Lang Suan.

Route number signs
DOH signs for public highways (ทางหลวง, thang luang) are white squares with a black garuda (ครุฑ khrut) centered above the route number. Signs near the beginning of a route may display the highway's name on a white rectangle above or below the square. Highways bypassing city centres bear the principal route number marked "Bypass" in Thai (เลี่ยงเมือง), and sometimes also in English.

Department of Rural Roads
DORR rural roads do not follow the regional numbering scheme, above. Signs may be black-on-white or gold-on-blue, with a two-letter province designation prefixed to the road number. Depicted is SK. 3015, for a rural road in Songkhla Province. The rural road network measures some 35,000 km, about 82 percent of which is paved. The Department of Rural Roads of the Ministry of Transport takes care of the maintenance of all the rural roads in Thailand.

Kilometer stones



 * Milestone as goal is lakh chai (หลักชัย); also see Lak Mueang, Lakh.

Lak or Lakh kilomet (หลักกิโลเมตร) single-carriageway kilometer stone facings display the route number on the outline of a garuda. Some kilometer stones also display the route number on top. Those located to the left of the carriageway display kilometers remaining to the road's beginning at kilometer 0. As seen on the right from the opposite lane, the kilometer stones ascend in value as one proceeds away from kilometer 0. On edges facing traffic, DOH kilometer stones usually show distances remaining to the next two towns, (amphoe seats, or provincial capitals.) Some edges, such as the one depicted to the left, have retroreflector panels. Dual carriageway kilometer stones or posts in the median strip show only the kilometer number.