Surprisingly enough, there are 74 other countries/regions in the world that drive on the left!!
That list includes the
U.S. Virgin Islands, along with many of the islands of the West Indies such as Jamaica and Dominica. Some places have switched sides in the past: Sweden changed from left to right in 1967, and some Canadian provinces drove on the left up until the 1920s.
Linking this to driving ability and tests though, I think people often place far too much emphasis on the left vs. right issue. I don't think it's that big a deal to start driving on the other side, odd incidents where somebody forgets and sets off on the wrong side of a quiet road notwithstanding.
I think it's often the other traffic rules and the general conditions which are more ingrained in our minds and take some getting used to. France's
priorité a droite rule is a good example. We know it exists, but for most British drivers it's a conscious effort to keep remembering that a 2CV might come hurtling out of a tiny side street to the right at any moment because it has priority.
And how many drivers here have you met who might have been thinking about visiting the United States for the first time and have no idea that a 4-way stop even exists, much less know the right-of-way rules at one?
To prevent somebody from another country from driving at all until he passes a local test would just not be practical. We have to assume that tourists and new residents have
some common sense, will realize that not all traffic rules, signs, etc. are the same everywhere, and will check up on the most important differences before setting off. (Again, look at all the travel books for France which explained the priority rules, the fact that -- at one time -- even visitors were expected to tint their headlights yellow, etc.)
Conversely though, I think the system needs to accept that experienced drivers arriving here already have the basic driving skills which would be applicable everywhere, and that it should not force them to sit the hazard-perception test (which is of dubious value anyway) and to drive in the specific, rigid fashion which is currently expected on the U.K. test.
Furthermore, if we allow drivers from countries as diverse as Greece, Finland, Estonia, and South Africa to simply swap a license, then there's no reason why American license-holders should not be treated similarly.