[kkielhofner]

Different country, smaller population, etc but Sweden did it in four years[0].

[0] - https://en.wikipedia.org/wiki/Dagen_H

[notahacker]

I don't think Sweden in 1963 had a comparable number of intersections designed with high speed merges in mind...

[sebzim4500]

The main difference there is that the vast majority of drivers already had LHD cars.