Big slow vehicles, like overloaded trucks, autoriskshaws, three wheel cargo vehicles, tractors, water carrying tractor trailers seem to like the right most lane to travel. This lane, in other countries, is called the speed lane, and is reserved for the fastest vehicles – On roads that do not have lanes, vehicles stick to the left side of the road (in countries that drive on the left. – In countries that drive on the right, vehicles stick to the right)
What happens because of this is that overtaking is very difficult. To overtake, vehicles coming from behind, do not see oncoming traffic properly. They will have to go right in the middle of the road, and sometimes to the right of the road to see oncoming traffic. This places them right in the path of oncoming traffic. No wonder that we see a lot of fatal head on collisions!

Otherwise, the fast car trying to overtake the slow vehicle going on the right of the road, will try to overtake the vehicle from the left – frustration leads to breaking of rules – right into traffic going on the left lane
If the slow moving vehicle moves in the correct lane, that is the left lane, overtaking would become much more simpler – the driver would easily be able to see if there is oncoming traffic, and traffic from behind. Needless to say, it would also be much less dangerous. 