And which came first? Was it a given that you drove your carriage on the left back in the seventeenth centuries? Did the American colonies continue this practice? When and why did the Americans change the side of the road they drove on?
Because.
How do you know the Brits didn’t swap sides of the road? 
The Thames was always drive on the left, as are most waterways since long before wide streets gave travelers much choice. That’s why your port side is on the left. It goes back before the Norsemen.
There didn’t need to be any laws about it when folks weren’t travelling at great speed or in great numbers. At some point governments had to make a decision and so they did.
One theory is that the Brits picked the left due to tradition, supposedly in Roman times you would walk/ride on the left so you could slice off the head of someone coming the other way if you didn’t like them with the sword in your left hand.
I just found an interesting webpage which some theories you may like to read
http://www.travel-library.com/general/driving/drive_which_side.html
I must confess that coming from a left hand side driving country I panic whenever I need to cross a road in the USA, it is much harder when you have to think about it. I also hate sitting on the wrong side of the car. I have been known to desperately go for the brakes as a passenger because if I am sitting on the right I must be driving. Just as well I don’t drive over there I think, I would be a menace
sword in your right hand
sheesh
I always thought there was also the added fact that when people rode horses, they traditionally mounted/dismounted on the left, rather than the right. It just made sense to ride/drive a horse/carriage/car etc on the left since that’s where it was “normal” to get off.
But I guess to easy cutting off people’s heads makes sense too… 
Sorry to nitpick, but the English drive on the left but have the steering wheel on the right, so they exit a car on the right-hand side.
I just returned from 3 months in India, and my impression was that there was little in the way of regulation regarding between left and right side driving. Perhaps they are in transition moving traffic from one side to the other??
What I get confused about is which side to you pass when strolling down a sidewalk and facing another pedestrian? It seems I often don’t get it right and have to make a quick swerve to avoid imminent physical contact. There seems to be a guideline here in Sydney to keep left but with so many tourists and immigrants (which includes myself) it’s just impossible to predict which way someone will swerve. I used to live in Asia where I found that pedestrians intentionally avoid eye contact when encountering this situation. It’s kinda like playing ‘chicken’ – you spot your target well in advance and then pretend your eyes are preoccupied and hope the other guy realizes that you are NOT going to swerve – and make him go around you. If he does, you keep face and he loses. Seems to me though that just as often both people crash into one another.
But back to traffic. What about planes and boats? When a ship sails from England to France which side does it keep to? Does it switch sides half way across?
Ships are guided by international treaty (since the ocean is international waters). Usually, you pass to the right.
What I want to know is how they paint the dotted line down the middle of the ocean.
Actually, the international nautical “rules of the road” are known as the COLREG’s. They are also modified in various local jurisdictions - in the continental US, the USCG “Inland Rules” apply. We could have a long, long discussion on either set.
(Sail has right of way over power unless the powered vessel has limited mobility - OK, what exactly is “limited”? In practice, it means you don’t get in the way of the Larkspur Ferry with your sailboat, whatever the rules say about your comparitive means of propulsion.)
The COLREG’s make provisions for approaching vessels to pass each other either port-to-port or starboard-to-starboard. Generally, port-to-port (keep right) is preferred, but in some situations a starboard-to-starboard passing is more desireable. The important part is that the vessels involved know each other’s intents, and the regs are very concerned with being able to signal those intents. I remember being told, in the context of pleasure boats, if you are about to collide head on with somebody who has ignored the normal signals and doesn’t seem to know what they’re doing, alter course to pass them port-to-port if at all possible, because staying to the right will be what they are used to from driving cars.
I’d imagine one of the main problems, even if England and others WANTED to transition to the “drive on the right” concept would be the pure cost of doing so. Most road markings would have to be repainted, signs redone, traffic signals moved, freeway on/off-ramps rerouted (maybe, I’ve haven’t thought that all the way through yet - maybe entrances would be become exits). Not to mention the safety implications of confusing almost every driver in the country.
It’s probably just too hard to change at this point even if they wanted to.
Sweden did it in 1967. It’s obviously something which has to be well-coordinated, since there is some point where everybody has to swap sides of the road. They spent months putting up new signage facing the other way, informing people, and having the actual switch occur at, IIRC, midnight on a specific day. Of course, they urged everybody to stay off the roads that night unless absolutely neccesary. I saw a photo of downtown Stockholm at “switch time” once - it was a total mess as many vehicles on a busy city street were trying to change sides.
it’s cos in old times you’re sword’s in your right hand so when you’re passing someone going the other way you have your sword in your best hand just in case.
Australia and Japan drive on the left too. also South Africa, India, Pakistan. Sweden used to but changed in the 1930s.
Well now that makes sense. I knew there was some flaw in my thinking - but years ago I had decided that it had to do with horses, and that was that :). Unless I just finally decided that it was all about the passengers - yeah, that’s it.
I think I’ll just go along with what Cecil says - seems the best thing to do.
I don’t know if this is true or not but here goes. Nigeria decided a few years ago to change over to driving on the right. One suggestion put forward, to make things easier, was that all buses and trucks would change over one week-end followed by the car drivers the next week-end. Sounds like an urban myth to me.
The answer really is quite simple…
The Bloody Poms have screwed up most everything else, so why not driving, too.
Consider the English language for example. It required the Americans to perfect the language to a point where you do not sound like a snobby snot when you speak it.
As with driving…
We drive on the right or correct side, the bloody poms use the wrong side.
Now see, wasn’t that simple? They are wrong, and we are right.
I’m told there are former British territories in the Carribean in which they drive on the left side, but the cars are American made, and thus have the steering wheel on the left as well!
You’re right, it would be too hard to do all at once, better if buses and taxis swap sides on, say, Monday, then motorbikes on Tuesday, goods vehicles Wednesday and finally cars on Thursday…
On a serious note though, I heard something to the effect that UK road junctions are now designed in such a way that they would work in reverse with minimal adjustment (except for signage and road markings), not sure if what I heard was true though.