It would be way more annoying for LHT countries to change to RHT than to switch from US units to metric, have to change literally every road sign, street light, road markings etc in the entire country.

Plus everyone would then be driving cars with the steering wheel on the wrong side for decades until the whole fleet got eventually replaced.

Countries have done it before, but I think there's a reason not many countries have done it since the mid 20th century when roads and cars were a lot simpler and there were fewer of them.

Switching to metric could be done gradually - replacing existing signs with metric ones only when they wear out, building new systems in metric while still using US units for the old ones.

There's no way to gradually switch from LHT to RHT, you have to somehow do a switchover in a day

> There's no way to gradually switch from LHT to RHT, you have to somehow do a switchover in a day

You could do it in phases. Have the trucks switch over on Monday, buses on Tuesday ...

And the US and various other countries (like the UK) effectively use hybrid systems for units for many purposes. Science and engineering in the US are heavily metric.

I used to think about whether we can get more countries switching back to LHT, but with Autonomous Vehicles that will no longer be much of an issue in 10 - 20 years time.

You'd also have things like bus doors now being on the wrong side (traffic-side), road curvature being wrong (e.g. motorway junctions and entry/exit ramps), let alone car mirrors and driver positioning.

> It would be way more annoying for LHD countries to change to RHD

As a complete aside, 1 country has done this: Sweden on 3 September 1967 and "It was by far the largest logistical event in Sweden's history"

https://en.wikipedia.org/wiki/Dagen_H