Cutting someone else's way is very common in Thailand. It's irresponsible from international perspective but "understandable" in Thai context because as soon as you appear in someone else's Range of Responsibility, he should heed you, even if you drive irresponsibly.
I believe that even the Thai police and Thai insurance companies make (implicit or explicit) assessments based on a driver’s Range of Responsibility, so that some accidents are blamed on the person behind while in an international context that person would have been right.
So the Range of Responsibility is really important and it actually conflicts with international traffic rules as you’re probably used to.
2. Adapt to each other rather than follow rules
Another important implicit feature of Thai traffic behaviour which has everything to do with Thai culture is that Thai drivers continuously adapt to each other rather than rely on real traffic rules. In other words, it’s more important to watch and adapt than to trust and follow the rules.
This collective implicit understanding in Thai traffic actually made me fail my Hong Kong motorcycle driving test, because I picked up this Thai habit of stopping or moving very slowly when the traffic situation appears unclear, while in international context you are supposed to rely on other drivers acting according to the rules and keep your speed.
Adapting to each other continuously is a typical Asian cultural feature that can be witnessed to its maximum in Vietnamese traffic, that consists of rivers of scooters flowing down the streets like a school of fish. Japanese people show adaptive yet at the same time extreme law-abiding behaviour, but like other Asians are also a group-oriented people where the harmony of the group are more important than the rights of an individual.
In stark contrast to that, Western people tend to be more strict and rely on rules while insisting on their individual rights, so here are some fundamental differences which clearly play out even in traffic situations.
Thai people aren't exactly amongst the most law-abiding people in the world, rules are publicly acknowledged but are often easily bent in public or broken in secret. In a similar way that many Thai treat a promise as being a mere intention that can be put aside using any (lame) excuse, rules, laws and regulations are often treated merely as guidelines that can be deviated from when you need to.
NOTHING REALLY MATTERS
EVERYTHING IS EMBRACED BY THE ALL-ABSOLVING MANTRA
THAT VIBRATES THROUGH THAI SOCIETY:
MAI-PEN-RAI SABAI-SABAI
In traffic, you can witness that in more cases than I can describe: parking right at corners (with red-and-white pavement signs), driving against the traffic flow, driving without lights on and so on. It all doesn't matter, it's is warmly embraced by the all-encompassing, all-absolving mantra that vibrates through Thai society: mai-pen-rai, sabai-sabai. Even by law enforcers.
For example, at intersections without traffic lights typically you can observe that normal traffic rules don't apply: traffic from the left is supposed to have priority, but in practice people just watch each others behaviour and the one who pushes on first often goes first. It is a constant give and take (sometimes more take than give), but in a more or less adaptive organic group process, rather than a strict demanding and judging based on rules.
Thai people don’t trust the rules, don’t rely on laws, and don’t trust each other, but instead they are flexible to the situation and adapt.
3. Hindering traffic flows is common
In Western driving schools you should have learned that you have to do what you can to avoid interrupting the flow of traffic, so you wait until cars pass before you park or get out of a parking space, you don’t stop at the side of the road when that will block the entire lane, you don’t drive slowly where you can drive a decent speed and you make turns quick enough. Much of this is nearly alien to Thai style traffic where the concept of avoiding hindrance to others is nearly absent. This is especially so for tuk tuk and red pick up trucks, which behave like a mafia anyway, but also for other uneducated road users.
Driving 20km per hour, making extremely slow turns, temporarily parking the vehicle in the middle the road thus blocking all traffic behind are all very common in Thailand.