Phetkasem Road (Thai: ถนนเพชรเกษม, rtgs: Phetchakasem Road or Thailand Route 4 (Thai: ทางหลวงแผ่นดินหมายเลข 4, AH2) is one of the four major highways in Thailand, along with Phahonyothin Road (Route 1), Mittraphap Road (Route 2), and Sukhumvit Road (Route 3). At 1,274 km, it is the longest highway in Thailand. It begins at Naowa Chamnian bridge in Bangkok Yai, Bangkok.