Reverse this if you live in Japan, the United Kingdom, or other countries where people deliberately drive on the wrong side of the road
In the United States the law is quite explicit: when there is more then one lane moving in one direction, highway drivers are supposed to drive in the right lane and reserve the left for overtaking-- passing in a word. Technically, the cops can write you a ticket for going slow in the left lane though they rarely do. Despite the law in many states it has become normal for people to drive in the left lane. The reason for that law is not ancient tradition. This was not original intent of our Founding Fathers. The reason for driving on the right and passing on the left is because if people actually observe that law, everyone is much more likely to get to their destination safely, and in a calm frame of mind.
Let me start by explaining why. Let us begin by assuming two lanes going in each direction. Slow drivers in the left lane block people who wish to pass. If they don't move over when they see someone coming, they form what racers know as a chicane, a kink in the course designed to slow people down. When a slow driver passes a very slightly slower driver everyone is impeded. Worse, when that slow driver does not return to the right lane, anyone wishing to pass must move left to get around the slow driver in the right lane then once past on the right lane, then move right to get around the slow car in the left. It's a classic chicane and the difficulty in doing that can line up many cars, wasting fuel and fraying nerves.
But the second reason for passing on the left is that it's simply safer, no matter how many lanes there are. This can be easily demonstrated by getting into the driver's seat of an automobile. First of all, cover up the mirror attached to your windshield. The reason for this is that most commercial vehicles and some private just don't have that mirror. Must of us use it heavily, because it's a great mirror, but the van I drive at work doesn't have one. No heavy truck or bus has such a mirror. The truth is that many of the vehicles you meet on the road don't have that mirror, so cover it so you understand what it's like not to have it. Then, once you've made certain that you have both the right and left outside mirrors properly aligned, compare the field of view found in each mirror. Note the "Objects in mirror are closer than they appear" message embossed on your right outside mirror. It's there for a reason.
In my work van, and pretty much every other vehicle I've driven, the view to the left side is ten times as large as the field of view on the right simply because the driver's eyes are much closer to the mirror. You see much more on your left. Most car drivers, spoiled by that lovely center mirror, rarely bother with the right hand mirror at all! But for trucks it is essential, yet offers only a restricted field of view. Now let's apply this perspective to the problem of passing. When passing another car, the overtaking driver can see the car being passed without effort. Both cars are moving forward, so drivers must look forward as well. Mirrors are checked only periodically. The driver of the overtaking car can clearly see the car being passed until the pass is nearly complete, by which time the car being passed almost always can he's being passed.
The problem is the overtaken car may not know they are being passed. The driver of that car checks his mirrors only periodically as he too must keep looking forward in order to avoid running into the school bus up ahead. Plus he has a restricted field of view to his rear. If you pass on his left, where his or her view is pretty good, the odds are good you will be seen. If you pass on the right and you're not in his mirror's "sweet spot" when he looks, the odds are pretty good you won't be seen. Not knowing you are there, he may move to the right into the space you intend to occupy.
The reason that you drive on the right is so that when you are being overtaken, you're much more likely to know about it and plan your moves accordingly. When you pass on the left you are much more likely to be seen, which makes it less likely the car you're passing may try to violate the laws of physics and cram two car bodies into the same space. When you enjoy a choice of passing on the right or left, choose the left when practical because you are much more likely to be noticed then if you pass on the right.
The roads are not the place for ideological battles or egotism. In the United States, Canada, most of Europe and lots other places, drive on the right and pass on the left. We'll all get their happier and safer.