Goverment must reduce amount of car on the street by increase gasoline cost (Maybe has too much effect on rest of society…), increase car tax, reduce amount of taxi, limit oldest car’s age which can be use on the street, etc.
Another important factor is the people, people must learn to drive properly and not park or stop their car where they shouldn’t, bus driver must not stop their bus on second or third lane. If people drive their car a lot more better, the traffic in bangkok will be surely a lot less severe than today.