Highway 21 (Phu Khae Intersection–Loei) is a major north-south national highway connecting provinces in Central Thailand to Phetchabun and Loei province. The route begins at Phu Khae Intersection in Chaloem Phra Kiat District, Saraburi Province, and terminates in Mueang Loei District, Loei Province, covering a total distance of 412.874 kilometers.
National Highway No. 21 falls under the jurisdiction of Highway Office 6 (Phetchabun) for a distance of approximately 325 kilometers แผนที่ระยะทางควบคุม สำนักงานทางหลวงที่ 6 เพชรบูรณ์ and Highway Office 11 (Lop Buri) for approximately 87 kilometers. แผนที่ระยะทางควบคุม สำนักงานทางหลวงที่ 11 ลพบุรี There are two bypass routes: the Phetchabun Bypass Road (National Highway No. 234) and the Lom Sak Bypass (National Highway No. 21 itself).
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