3.1 Longitudinal Road Markings & Centerlines

3.1 Longitudinal Road Markings & Centerlines

Quick Summary: Longitudinal pavement markings run parallel to traffic flow, establishing mandatory boundaries between lanes and opposing traffic directions under Thailand's Land Traffic Act B.E. 2522. As a core rule: white lines separate traffic traveling in the same direction, yellow lines divide opposing traffic streams, solid lines prohibit crossing or lane changing, and broken lines permit maneuvers when safe. Red-and-white curbs prohibit all stopping and parking; yellow-and-white curbs ban parking but permit rapid passenger drop-off; and white-and-black curbs serve as road-edge guides where parking is generally allowed.

Longitudinal road markings are the foundational visual infrastructure of road safety. Under the Department of Land Transport (DLT) theory examination standards and the Land Traffic Act B.E. 2522 (1979), drivers are legally bound to obey all markings painted on road surfaces with the same strict compliance required for elevated physical road signs and electrical traffic signals. Longitudinal markings run parallel to the direction of vehicle travel. Understanding the precise legal significance of line colors, continuous solid lines, broken dashed patterns, and curb edge markings is essential for accident prevention and passing the DLT licensing test.

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1. Lane Markings for Same-Direction Traffic (White Lines)

On multi-lane roadways where multiple lanes travel in the identical direction, lane boundaries are demarcated using white pavement paint. The continuity of the white line communicates whether a driver is permitted to transition between lanes.

Broken White Lane Lines (เส้นประสีขาว)

A broken white line is made of short painted segments with gaps between them.

• Legal Meaning: Separates lanes of traffic traveling in the same direction. Lane changes, lane merging, and overtaking are legally permitted.
• Mandatory Driver Protocol: A driver may only change lanes over a broken white line after conducting a complete defensive safety routine:
  1. Check interior rearview and exterior side mirrors to assess surrounding traffic flow.
  2. Activate the appropriate turn indicator signal at least 30 meters in advance of initiating the maneuver.
  3. Perform a physical head-check (blind spot shoulder check) to verify that no motorcycle or vehicle is traveling in the vehicle's rear quarter blind spot.
  4. Smoothly steer into the adjacent lane without disrupting the speed or headway of following vehicles.

Solid White Lane Lines (เส้นทึบสีขาว)

A continuous, unbroken white line painted between adjacent lanes moving in the same direction.

• Legal Meaning: Strictly prohibits lane changes, overtaking, and crossing. Drivers must remain entirely within their current lane until the marking transitions back to a broken line.
• Deployment Locations: Solid white lane lines are systematically applied at locations with elevated collision risk, including:
  • Approaches to signalized intersections and roundabouts (channelization zones).
  • Inside tunnels, underpasses, and elevated expressways.
  • Across bridges and narrow viaducts where lateral escape space is non-existent.
  • Approaches to pedestrian crosswalks and railway level crossings.
  • Merging and diverging points near highway on-ramps and off-ramps (gore areas).

Combination White Lane Lines (เส้นประคู่เส้นทึบสีขาว)

A combination line features a solid white line running immediately alongside a broken white line between same-direction lanes.

• Legal Rule: Drivers traveling in the lane adjacent to the broken line are permitted to change lanes when safe. Drivers traveling in the lane adjacent to the solid line are strictly prohibited from crossing into the adjacent lane.
• Common Application: Frequently painted on expressway entry deceleration and acceleration lanes to allow entering vehicles to merge outward while preventing expressway through-traffic from cutting abruptly into exit corridors.

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2. Centerlines Separating Opposing Traffic (Yellow Lines)

On two-way roadways where oncoming traffic flows in opposite directions without a physical concrete median barrier, centerlines are painted in yellow. Yellow centerlines carry immense legal weight under Thai law because crossing into an oncoming traffic lane presents severe head-on collision risks.

Broken Yellow Centerline (เส้นประสีเหลืองแบ่งทิศทางจราจร)

A single dashed yellow line running down the geographic center of a two-lane, two-way roadway.

• Legal Meaning: Divides opposing traffic flows. Drivers are legally permitted to cross the centerline temporarily into the oncoming lane solely to overtake slower-moving vehicles, or to execute a legal right-hand turn into an intersecting soi or driveway, provided the maneuver can be completed with absolute safety.
• Safe Overtaking Requirements: Before crossing a broken yellow centerline:
  • The driver must have a long, clear view of the road ahead. At highway speeds, an overtake needs several hundred metres of clear road.
  • The oncoming lane must be completely clear of approaching vehicles.
  • The vehicle in front must not be signaling to turn or overtaking another vehicle.
  • The maneuver must be completed and the vehicle returned to the left-hand lane well before any oncoming vehicle approaches within safe braking distance.

Solid Yellow Centerline (เส้นทึบสีเหลืองเดี่ยว)

A single continuous yellow line down the center of a two-way road.

• Legal Meaning: Strictly prohibits crossing or straddling the centerline for overtaking.
• Hazard Context: Deployed where forward sight distance is restricted, such as winding rural roads, blind curves, crests of hills, approaches to bridges, and obstructed roadside view corridors. Drivers must remain strictly on their side of the road.

Double Solid Yellow Lines (เส้นทึบคู่สีเหลือง)

Two parallel, continuous solid yellow lines running down the center of an undivided multi-lane or high-hazard roadway.

• Legal Meaning: Represents an absolute, impassable legal barrier separating opposing traffic streams.
• Statutory Enforcement: Crossing, straddling, or making a U-turn across double solid yellow lines is classified as a severe moving violation under the Land Traffic Act B.E. 2522. Crossing the centre line to overtake is an offence under section 47 of the Land Traffic Act, and you carry heavy civil and criminal liability if a crash follows. Never cross double solid yellow lines under any normal driving circumstance.

Combination Yellow Centerlines (Solid and Broken Side-by-Side)

A pair of yellow centerlines where one line is solid and the other is broken.

• Asymmetric Legal Privilege:
  • Vehicles traveling on the side of the broken yellow line may cross the centerline to overtake slower vehicles ahead when the road is clear.
  • Vehicles traveling on the side of the solid yellow line face an absolute prohibition against crossing or overtaking.
• Engineering Purpose: Typically implemented on roads approaching a curve or hill crest. Traffic ascending a hill or entering a blind curve faces the solid line (no visibility, no overtaking), while traffic descending the crest or exiting the curve faces the broken line (open sightlines, overtaking permitted).

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3. Road Edge Lines and Shoulder Demarcation (เส้นขอบทาง)

A continuous solid white edge line is painted along the outer boundary of the carriageway to demarcate the edge of the active traveled lane from the road shoulder (ไหล่ทาง).

• Legal Status of the Shoulder: The shoulder is not a travel lane. Using it to pass slower traffic counts as illegal passing on the left, because section 45 allows that in only two cases. Using it to jump a queue is a classic example of the bad manners the DLT tests.
• Authorized Shoulder Use: The paved or gravel shoulder is reserved exclusively for:
  • Emergency vehicle stopping due to mechanical breakdowns or flat tyres.
  • Ambulances, police, and emergency response vehicles responding to incidents.
  • Drivers directed onto the shoulder by a uniformed traffic police officer.
• Safety Hazard: Motorcyclists, agricultural machinery, and broken-down vehicles frequently occupy shoulders; overtaking on the shoulder causes severe rear-end impacts.

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4. Curb and Gutter Markings (เครื่องหมายจราจรบนขอบทางเท้า)

In urban municipalities across Thailand, parking and stopping regulations are indicated directly by paint applied to the vertical face and horizontal top surface of roadside concrete curbs. Candidates must memorize the three standardized curb marking color codes.

Red-and-White Stripes (แถบสีขาวสลับแดง)

• Legal Meaning: Strictly prohibited to STOP or PARK at all times (ห้ามหยุดและห้ามจอดรถทุกชนิดตลอดเวลา).
• Driver Obligation: No vehicle of any classification may come to a halt alongside a red-and-white curb for any duration. Even stopping momentarily to allow a passenger to jump out or to pick up a parcel is an illegal act that subjects the driver to immediate traffic citations, wheel-clamping, or towing.
• Typical Locations: Bus stops, road corners, within 10 meters of an intersection, pedestrian zebra crossings, fire hydrants, bridge portals, and traffic choke points.

Yellow-and-White Stripes (แถบสีขาวสลับเหลือง)

• Legal Meaning: NO PARKING zone (ห้ามจอดรถ), but TEMPORARY STOPPING is permitted (หยุดรับ-ส่งผู้โดยสารได้ชั่วคราว).
• Driver Obligation: A driver is legally permitted to halt alongside a yellow-and-white curb solely for the immediate alighting or boarding of passengers, or for the rapid loading or unloading of goods, provided:
  • The driver stays with the vehicle and in control of it.
  • The stop is momentary and does not disrupt oncoming or passing traffic.
  • The vehicle departs immediately once the passenger has entered or exited.
  • Leaving the vehicle unattended or waiting for passengers alongside a yellow-and-white curb constitutes illegal parking.

White-and-Black Stripes (แถบสีขาวสลับดำ)

• Legal Meaning: Curb Guide Marking (เครื่องหมายแสดงแนวขอบทาง).
• Driver Obligation: Alternating black-and-white curb segments are visual contrast markings designed to highlight the presence and physical profile of elevated curbs, traffic islands, bridge abutments, and road dividers, especially under night driving conditions.
• Parking Rules: Parking alongside a white-and-black curb is generally permitted, provided that no separate regulatory sign (such as a No Parking sign or odd/even date parking sign) prohibits it, and the vehicle is parked within 25 cm of the curb without violating general statutory rules (such as blocking driveways or hydrants).

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5. Summary Reference: Longitudinal & Curb Markings