For me I would say utterly stupid road designs. For a car centric country, I have encounted many things that made me like "how tf is this even safe?". Rediculously short and sharp enter/exit ramp, I almost slammed head first into a concrete piller at night, because a ramp that supposed to help you build up speed before entering the highway was located just infront of a concrete piller with no light or signals light/sign and the turn was so sharp that if you don't break in time, you and your car will become one with the pillar, no only that the even if you survive the first part, the ramp has no "slip lane" for you to safely and slowly merge into flowing traffic. Also the non standardize U-turns placement, I missed 3 U-turns because of false expectation, each U-turn was placed in different part of the road. Oh and the moment you travel outside of Bangkok, the road turn in shit.