ถนนเพชรเกษม Thanon Phetkasem | 
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| Part of  AH2  AH123 | 
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| Existed | 1950–present | 
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| North end | (Starting from Phetkasem Road) Naow Chamnian Bridge in Bangkok Yai, Bangkok (Beginning of Highway 4) Phetkasem Road, km.26+420 in Om Yai, Sam Phran, Nakhon Pathom | 
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| Major intersections |  in Bang Khae, Bangkok  in Nakhon Pathom (Planned)  in Nakhon Chai Si, Nakhon Pathom  in Nakhon Chai Si, Nakhon Pathom  in Mueang Nakhon Pathom, Nakhon Pathom  in Ban Pong, Ratchaburi  in Bang Phae, Ratchaburi  in Mueang Ratchaburi, Ratchaburi  in Mueang Ratchaburi, Ratchaburi  in Mueang Ratchaburi, Ratchaburi  in Pak Tho, Ratchaburi  in Thayang, Phetchaburi  in Cha-am, Phetchaburi  in Pranburi, Prachuap Khiri Khan  in Mueang Prachuap Khiri Khan, Prachuap Khiri Khan  in Thasae, Chumphon ,   in Mueang Chumphon, Chumphon  in Mueang Ranong, Ranong  in Takua Pa, Phang Nga  in Takua Thung, Phang Nga  in Ao Luek, Krabi  (Planned)/ in Ao Luek, Krabi  in Huai Yot, Trang  in Mueang Trang, Trang  in Mueang Trang, Trang  in Mueang Trang, Trang ,  in Mueang Phatthalung, Phatthalung  in Rattaphum, Songkhla  in Bang Klam, Songkhla  (Planned)/
,  in Hat Yai, Songkhla  in Hat Yai, Songkhla (Planned)  in Sadao, Songkhla  (Planned)/ in Sadao, Songkhla | 
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| South end |  FT 1,  E1 in Sadao Checkpoint, Sadao, Songkhla | 
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| Country | Thailand | 
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| Provinces | Bangkok, Samut Sakhon, Nakhon Pathom, Ratchaburi, Phetchaburi, Prachuap Khiri Khan, Chumphon, Ranong, Phang Nga, Krabi, Trang, Phatthalung, Songkhla | 
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| Major cities | Bangkok, Nakhon Pathom, Prachuap Khiri Khan, Chumphon, Ranong, Phang Nga, Krabi, Trang Phatthalung, Hat Yai, Sadao | 
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Phet Kasem Road (Thai: ถนนเพชรเกษม, RTGS: Thanon Phet Kasem, pronounced [tʰā.nǒn pʰét kā.sěːm]) or Highway 4 (Thai: ทางหลวงแผ่นดินหมายเลข 4,  AH2) is one of the four primary highways in Thailand, along with Phahonyothin Road (Highway 1), Mittraphap Road (Highway 2), and Sukhumvit Road (Highway 3). At 1,310.554 km, route 4 is the longest highway in Thailand.