The British drive on the left; Americans drive on the right. Simple enough. Since you can’t drive directly from here to there, you don’t have to worry about switching in mid-road somewhere. The Channel Tunnel goes from England to France (where they drive on the right), but you don’t actually drive through the tunnel, so no lane-switching problems ensue. If you look at a list of all the places in the world where they drive on the left, you’ll see that it’s a fair indication of the former extent of the British Empire. Islands like New Zealand and Sri Lanka, like England itself, can be self-contained zones of left-driving. But what about India? Left-driving India is connected by the Eurasian landmass to right-driving France. If you get in a car in Hyderabad and drive to Marseilles, somewhere you have to switch. What is that like?
I found the answer on an excellent site called Which side of the road do they drive on? Put together by Brian Lucas, it even has a world map that reveals the thing I was curious to see: there are enormous frontiers between countries that drive on opposite sides. At the Khyber Pass on the border between Pakistan and Afghanistan, for example, you have to switch sides. What are these border crossings like? Lucas is kind enough to have compiled some answers.
It’s so satisfying to be puzzled about an obscure topic and find an extensively documented well-maintained web page about that very thing.
Rambles Blog at Star Chamber 
Dear Ned — Having lived in the UK now for 6 months, and travelled to 8 European countries in that time, this post (and related links) has brought bountiful joy and enlightment. Many thanks! Personally I found it very helpful, when renting a recently car at the airport in Edinburgh, Scotland, that all exits from the parking lot sport large signs reminding “Drive on the left” (in several languages). Still, as an American used to driving on the right, it takes a while to get a feel for how much vehicle you have to your left here. On my way back to return the car, driving south down a narrow road along the west bank of Loch Lomond, I rather nailed the passenger side rear view mirror into a stone embankment. (I haven’t seen a bill from them yet!) – Joel
Having been in India recently, I can tell you that they drive on both sides of the road.