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, Thai: 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.[1] 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.[2] Some upgraded sections of highway are being turned into a "motorway", while other motorways are not being built from highway sections. See Thai motorway network.
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The 1992 Highway Act (Thai: พระราชบัญญัติทางหลวง พ.ศ. 2535), revised as the 2006 Highway Act (Thai: พระราชบัญญัติทางหลวง (ฉบับที่ 2) พ.ศ. 2549), defines the following five highway types:[3]
A special highway (Thai: 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 (Thai: 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 (Thai: 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 (Thai: 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 (Thai: 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.
The first digit of a highway number indicates the region of Thailand it primarily services:[4]
The number of digits in a highway number indicates the classification of the highway.
A single digit number indicates a principal highway connecting between regions. There are four of these:[4]
A two-digit number indicates a principal highway within a region. For example, Route 22 is a main highway in the northeast that runs between Udon Thani and Nakhon Phanom.[4]
A three-digit number indicates a secondary highway. For example, Route 202 is a secondary highway in the northeast that runs between Chaiyaphum and Khemarat; Route 314 is a secondary highway in the central region, between Bang Pakong and Cha Choeng Sao.[4]
A four-digit number indicates an intra-province highway connecting the provincial capital to the districts, or to important sites in the province. For example, Route 1001 is a northern highway that runs between Route 11 Intersection and Amphoe Phrao; Route 4006 is a southern highway that runs between Route 4 Intersection (Ratchakrut) and Lang Suan.[4]
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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 principle route number marked "Bypass" in Thai (เลี่ยงเมือง), and sometimes also in English.
DORR rural roads do not follow the regional numbering scheme, above. Signs are gold-on-blue, with a two-letter province designation prefixed to the road number. Depicted is YS.4011, for a rural road in Yasothon 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.[5]
Lak or Lakh kilomet (หลักกิโลเมตร) single-carriageway milestone facings display the route number on the outline of a garuda; some milestones also display the route number on top. Those located to the left of the carriageway count kilometers left to the road's beginning at KM 0; as seen on the right from the counter lane, the milestones go right on counting. On edges facing traffic, DOH milestones 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 milestones or posts located in the median show only the kilometer number.
DORR milestones show the kilometer number, and the edges may show distances remaining to the next two villages.
Older roads built by the Ministry of the Interior Public Works Department (กรมโยธาธิการ กระทรวงมหาดไทย) have only department insigne and kilometer number, and do not show distances on their edges.
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