Distribution of the type of vehicles in Bangkok and the rest of Thailand, explains partly the graph below.
You can see that there are about the same number of traffic accidents (2012) in Bangkok and in the rest of the country all provinces combined. In Bangkok, proportionally more personal cars are involved in accidents, while in the 'countryside' pickups feature more prominently. Taking into account the number of motorcycles in Bangkok and the whole country, there are proportionally more accidents involving motorcycles in the capital.
In any case, the number of accidents in Bangkok itself when compared with the whole of the country, is rather striking. Maybe there is much more reporting of traffic accidents in the capital. After all, there is substantial pressure, each time even small accidents occur, because drivers want the streets cleared (and traffic resumed), which often necessitates police involvement.