Basement Geographer:

Generally, left-driving countries are isolated from right-driving countries by water, mountains, desert, or large stretches of deep forest. While this provides for relatively little traffic crossover between the two mode, there are still numerous roads (about 86 or so) where the issue of having to exchange lanes due to crossing over a border exist. These occur mostly in Africa and Asia, along with a handful of roads leading out of Guyana and Suriname in South America. To ease the transition for drivers so that they don’t suddenly find themselves hurtling headlong into oncoming traffic, there are a number of ways of handling the issue.

This is an old article but it talks about something I’ve never thought about. I’ve driven in countries that are right-hand drive but never had to cross a border and make the described transitions.