The reason the traffic can’t move is because there are too many cars for the amount of road area. Building new roads won’t change that, because more people will just buy more cars. The answer is trains and subways. Not just a line here and a line there, but a complete subway system that reaches all of Bangkok with a complete train system that does the same, overlapping it.
There are many cities less developed than Bangkok that have had complete systems for years. Yangon has had a circle line for decades. Kolkata has had streetcars for so long that it is amazing they are still functional. Of course the Bangkok trains would need to link to an affordable 1.4-metre-gauge system extended across the whole country. Once that is achieved, anyone who still wanted to sit bumper to bumper in road traffic would still have that option.
Charlie Stampfer