Minimum speedThailand
3. Minimum speed
A minimum speed is the lowest speed that vehicles are required to maintain on a particular road or under specified conditions. Minimum-speed rules are intended to prevent vehicles travelling unnecessarily slowly from disrupting traffic or creating large speed differences between road users.
A minimum speed does not require drivers to maintain that speed when traffic, weather, visibility, road conditions or safety require them to slow down. The way minimum speeds are indicated and enforced varies between countries.
Wikimedia.org • CEphoto, Uwe Aranas • CC BY-SA
Is 100 km/h the minimum on every Thai highway?
No. The 100 km/h minimum for ordinary cars in the rightmost lane belongs to qualifying, designated highways allowing 120 km/h. Exceptions apply where traffic, visibility, an obstruction or another difficulty makes that speed impracticable. It is not a blanket minimum for every highway, motorway or lane; lower signs and safe-driving requirements still apply.
🔗 infocenter.oic.go.th • OIC official summary — 2021 speed frameworks and vehicle/road categories
🔗 upload.wikimedia.org • Royal Gazette facsimile of full 2021 general speed Regulation, mirrored PDF
🔗 ratchakitcha.soc.go.th • Royal Gazette — full December 2024 Bangkok speed and horn order
Start of a minimum speed.
English • Traffic sign บ.54 • Minimum speed: 30 km/h. Sets a 30 km/h minimum on the controlled lane or section. Take traffic conditions and safety into account.
ไทย • บ.54 • ความเร็วขั้นต่ำ 30 กม./ชม.. ป้ายนี้กำหนดความเร็วไม่ต่ำกว่า 30 กิโลเมตรต่อชั่วโมงในช่องทางหรือช่วงทางที่ควบคุม ให้คำนึงถึงสภาพการจราจรและความปลอดภัยด้วย
Sources
🔗 Commons.wikimedia.org
🔗 Doh.go.th • กรมทางหลวง • คู่มือ พ.ศ. 2561 • เล่ม 1 • หน้า PDF 85
SVG source: Government of Thailand • Public domain • Original revision 2017-11-12. Wikimedia original unmodified.

Traffic rules
Thailand