fulgurafrango [blog]
20 December 2011


A two-by-two version of a “coordination game” goes as follows: two agents prefer different activities but would still rather do things together than do things alone—even to the extent of engaging in the less preferred activity. The most common example is the “battle of the sexes” (he wants to go to a boxing match; she wants to see a musical, but neither wants to go solo.)

A less trivial example, however, is the practice of driving on the left-side or on the right-side of the road. The payoffs are as given in Figure 1: A would rather drive on the right and B likes driving on the left. (The numbers in parentheses (a, b) are the payoffs to A and B, respectively. If both A and B drove on the left, for example, then A‘s payoff would be 3 and B‘s would be 5.) Obviously, however, it would be disastrous if each did what he wanted, since that would lead to major accidents. They would both be better off if they drove on the same side of the road.







(5, 3)

(-2, -1)


(-1, -2)

(3, 5)

Figure 1

So ultimately either both will drive on the left, or both will drive on the right—there are no theoretical or a priori reasons, however, for a particular solution to be selected. In game theory terms, there are said to be multiple (in this case, two) pure Nash equilibria. History or pure chance are then said to play a role in the actual choice.

But how was the choice of which side of the road to drive on historically arrived at? Some important elements in the selection appear have been: (a) the fact that most people are right-handed; and (b) the peculiar requirements imposed by different ways of driving animal-drawn vehicles which were in use at the time these conventions were arrived at.


There were basically two ways of driving a horse-drawn coach or carriage. The first was for a driver (cochero or kutsero) to sit in front of the passengers (on what was known as a “box seat”) and to control the horses in front. Let’s call this the coachman system. It is important to note that in Pictures 1 and 2, the driver in this system tends to seat himself towards the right side of the carriage. The reason is that most drivers are right-handed, and they will hold the whip with their (strong) right hand and the reins with the left. If they sat towards the left side instead and continued to wield the whip with their right hand, there would be a risk of hitting their passengers seated behind them, or of the whip becoming tangled in the reins.

Picture 1

Picture  2

The second—less familiar—way to drive a coach, however, is for the horses to be ridden and guided directly. This is called the postilion system. This is said to have first arisen in Napoleonic France, when cargo wagons were pulled by teams of horses. In this mode, there is no need for a front-seat driver. This can be seen in pictures of the carriages in Britain’s royal weddings (Pictures 3 and 4). (Note that in Picture 4, the two men in red livery behind William and Kate are not driving the carriage; they are footmen.) An apocryphal story from Spain says that box-seated coach-drivers were eliminated and the postilion system was adopted after an incident involving a coachman who eavesdropped on his high-ranking passengers, revealing state secrets and causing the government great embarrassment.

Picture 3

Picture 4

The “drivers”in the postilion system—also called the postilions since the mail service adopted this system—are the men in front riding the left-side horses, i.e., the left-rear or the left-front horses, or in the case of Picture 4, both. There are two reasons postilions need to ride the horses on the left. First, the postilion is usually right-handed and needs to guide the horse on the right using his good right hand. Second, most horses are used to being mounted only from the left side, and it is impossible to do this other than with the horses on the left side .

As it turns out adopting one or the other type of coach-driving is a major reason that people tend to drive on either the left side or on the right side of the road.

If, as in the postilion method, you are on the left side of the carriage, you will want to keep to the right side of the road. This is because if there is any on-coming traffic, you will want to see the clearance between yourself and the approaching vehicle.

Figure 2

In Figure 2 above, the dark spots are the drivers positioned on the left side of the carriage because they are driving the leftmost horses. They can best judge the distance between their vehicle and oncoming ones by driving on the right side. By contrast, if you are driving from the right side of the carriage (as in the coachman system), you will want to drive on the left side of the road for the same reason.

So the system of coach-driving is one of the main reasons cited for people driving on one side of the road or another.[1] (As another consequence, note that if it has been settled that you must keep to the right side of the road, then people will want to put the controls on the left side, as automobile manufacturers like Ford have done since the early days.)


A final addendum to this piece has to do with the etymology of some curious Filipino terms. You may want to consult your grandparents, but there was a time Filipinos (at least in urban Manila) used the term mano to mean “right”, and silya to mean “left”.  In instructing a taxi driver, they would not say, kaliwa diyan; rather they would say silya, mama; and to turn right, they would say mano diyan. If you wanted the cab to veer to the right sidewalk, you would say karga de mano. And to veer to the left sidewalk (which was illegal but possible), you would say karga de silya. The original Spanish words were obviously silla (= “chair” or “saddle”) and mano (= “hand”).

There are at least two possible derivations of these terms, both related to the postilion method of driving carriages. As already discussed, if horses were harnessed two abreast, the left one would have a saddle on which the postilion rode—hence caballo de silla. The horse on the right, on the other hand, would be unsaddled but guided by the hand or the whip—hence caballo de mano. From this derives the association of silya with “left” and of mano with “right”. By extension, karga de mano means “veer as if to load something on the right side of the vehicle”, while karga de silya means do the same thing but on the left side of the road. These postilion-related terms were in use much earlier than taxicabs, of course, from which one may deduce that carriages in the Philippines tended to be driven on the right side of the road.

The other possibility is that silya could have referred to the coachman style of driving itself, while mano referred to the postilion style. As already discussed, however, vehicles tend to be driven on the left-side of the road in the coachman style, while one tends to drive on the right side of the road in the postilion style. Hence, again, silya = “left” and mano = “right”.


[1] Among other primeval explanations was that when walking, a right-handed man with a sword would want keep his sword-arm free to the maximum extent to defend himself, so he would keep to the left side of the road and let other people pass him on his right. Another explanation was that when mounting a horse, a right-handed person would find it easiest to mount it from the left side of the horse. These explanations make left-side driving or riding more natural.