Thai law says you overtake on the right, and that passing on the left is prohibited except in defined circumstances. Thai roads say that being passed on both sides simultaneously is an ordinary Tuesday. Both statements are true, and a visiting driver needs to hold both of them at once: obey the rule yourself, and never assume anybody else is.
The specific danger for a foreign driver is not being overtaken. It is the two-lane highway overtake, where a vehicle coming the other way pulls out to pass a truck and expects you to move onto the hard shoulder to make room. That is a normal and widely understood manoeuvre here. If you do not know it is coming, it looks like a head-on collision developing.
- Overtaking sideThe right
- Passing on the leftProhibited, with exceptions
- Banned near a junction or crossingWithin 30 metres
- Maximum penalty400–1,000 baht (s.157)
The rule as written
Section 44 requires an overtake to be made on the right, leaving an appropriate space, with a return to the left afterwards. Section 45 prohibits overtaking on the left, with two exceptions: where the vehicle ahead is turning right or signalling a right turn, and where the road has two or more lanes in the same direction — and in the second case only if no vehicle is following behind you.
Section 46 is the list of places where overtaking is banned outright, and it is more detailed than most countries' equivalent:
Where overtaking is prohibited (section 46)
- Uphill on a steep slope, on a bridge, or in a curve — unless a sign permits it
- Within 30 metres of a pedestrian crossing, a junction, a roundabout, a traffic island or a railway level crossing
- Where fog, rain, dust or smoke reduces visibility below 60 metres
- At a critical point or in a safety zone
Section 47 prohibits crossing the centre line to overtake. Section 48 prohibits overtaking through a bus lane. Breaches of sections 45 to 48 carry 400 to 1,000 baht under section 157 as the Act was originally drafted.
The 60-metre visibility rule in section 46 is worth remembering during the rainy season. A tropical downpour reduces visibility below 60 metres routinely and without much warning, which means that in a heavy shower overtaking on a two-lane road is not merely unwise but prohibited. The rainy season and flooded roads covers what those conditions are actually like.
What actually happens: undertaking
On any Thai multi-lane road you will be passed on the left. Constantly. By cars, by pickups, by buses, and by motorbikes riding in the space between your car and the vehicle beside you.
This is not lawlessness so much as a different organising principle. Thai lanes sort themselves by vehicle type and destination more than by speed, so the outside lane is not reserved for faster traffic and the inside lane is not reserved for slower traffic. A vehicle that wants to pass will pass on whichever side has room. Lane discipline in Thailand explains the pattern in full; the consequence for overtaking is that a mirror check is not optional and a shoulder check is not optional either.
Never change lane on mirrors alone. A motorbike doing 70 km/h in the gap between lanes does not appear in a door mirror until it is level with you. Look over your shoulder, every time, in a car and on a bike. This is the manoeuvre that produces the most common serious collision between a foreign driver and a Thai rider.
The two-lane highway overtake
Much of rural Thailand is single-carriageway: one lane each way, a wide sealed shoulder on both sides, and a great deal of slow traffic — trucks, agricultural vehicles, songthaews, farm machinery and motorbikes carrying implausible loads.
The local convention is well established. A vehicle overtaking pulls into the oncoming lane. Oncoming traffic moves onto its own hard shoulder to give it room. Three vehicles briefly occupy a two-lane road, and it resolves. Drivers here flash headlights or use the horn to announce the manoeuvre rather than to object to it — see horn and headlight etiquette on Thai roads, because a flash in Thailand usually means "I am coming through", not "after you".
How to handle it from your side:
- Watch the oncoming lane as far ahead as you can see, not just the vehicle in front of you. The overtake that concerns you starts several hundred metres away.
- Move to the left within your lane as soon as you see it developing. You do not usually need the shoulder; you need to be visibly making room early.
- Check the shoulder before you use it. It contains motorbikes travelling in both directions, parked pickups, dogs, and occasionally a stationary vehicle with no lights.
- Do not brake hard. Whatever is behind you has not seen what you have seen.
- Do not flash your lights to object. It will be read as an invitation.
Overtaking a truck yourself
The specific problem is that Thai trucks are often slower than they look, frequently overloaded, and sometimes have no working lights at all. Getting past one requires more road than the same overtake at home.
Pull back before you commit. Sitting close behind a truck gives you no view of the road ahead and no run-up. Drop back three or four vehicle lengths, look through and around the truck, and only then move out. If you cannot see the road clearly beyond it, you are not in a position to overtake it.
Be aware that a truck driver may wave you past or flash a left indicator to signal that the road ahead is clear. Many drivers do this and mean it. It is a courtesy, not a guarantee, and the responsibility for the manoeuvre remains yours. Do not accept an invitation into a blind section.
If the truck ahead of you is doing 70 km/h and your journey is 200 kilometres, overtaking it saves you perhaps eight minutes. On a two-lane Thai highway in the dark, in the rain, or on a hill, that is not a trade worth making. There will be a dual-carriageway section soon enough.
Overtaking on a motorbike
Riders in Thailand pass on both sides continuously, and a great deal of the time nobody minds. There are three things worth being deliberate about.
First, filtering forward at a red light is not addressed by the Act as a separate offence and is not something police stop riders for. That is different from saying it is regulated and permitted — it is simply not squarely covered. See motorbikes, filtering and Thailand's left lane.
Second, passing a long vehicle on the left puts you in the exact position where the driver cannot see you and where the vehicle will move if it turns left. Pass on the right if you have any choice.
Third, a rental scooter does not have the acceleration to complete an overtake you would attempt on a bike at home. A 110cc or 125cc automatic with a passenger has very little in reserve above 70 km/h. Judge the gap on what the bike will actually do, not on what your instincts from a larger machine say.
What it costs if you get it wrong
The statutory penalties for improper overtaking are modest: 400 to 1,000 baht under section 157, roughly £9 to £22. As with nearly every Thai traffic penalty, that is a maximum range rather than a fixed charge, and a roadside ticket is normally at the lower end. Traffic fines in Thailand sets out how the ticket amounts actually work.
The number that matters is not the fine. Overtaking errors on two-lane roads produce head-on collisions, and a head-on collision at combined speeds above 150 km/h is the kind of event that medical evacuation insurance exists for. Reckless driving is a separate and far more serious offence, carrying up to a year and a fine of 5,000 to 20,000 baht since the 2022 amendment.
Frequently asked questions
Which side do you overtake on in Thailand?
The right. Section 44 requires overtaking to be done on the right with an appropriate gap, followed by a return to the left. Passing on the left is prohibited by section 45 except where the vehicle ahead is turning or signalling right, or where the road has two or more lanes in the same direction and nothing is following you.
Is undertaking legal in Thailand?
Generally no, though the multi-lane exception in section 45 makes it lawful in more situations than a British driver would expect. What matters more is that it happens constantly regardless, so you must check your near side before every lane change.
Where is overtaking banned?
Uphill on a steep slope, on a bridge and in curves unless signed otherwise; within 30 metres of a crossing, junction, roundabout, traffic island or level crossing; at critical points and in safety zones; and whenever fog, rain, dust or smoke cuts visibility below 60 metres.
Why do oncoming cars pull out towards me?
They are overtaking, and the convention on two-lane Thai highways is that oncoming traffic moves left onto its shoulder to make room. Move left early and steadily, check your own shoulder before using it, and do not brake hard.
What does it mean when a truck flashes its indicator at me?
Usually that the driver believes the road ahead is clear for you to pass. Treat it as information rather than authorisation — you cannot see what they can see, and the responsibility for the overtake stays with you.
What is the fine for illegal overtaking in Thailand?
Sections 45 to 48 carry 400 to 1,000 baht under section 157 as originally drafted. That is a statutory range, not a fixed roadside charge. Where an overtake is genuinely dangerous, the offence charged may instead be reckless driving, which since 2022 carries up to a year and a fine of 5,000 to 20,000 baht.