You know how in other countries the signs look different from the ones in America and very similar or exactly the same as they do in Europe? And the yellow light before the light turns green? Yeah, that’s because of this:
The Convention on Road Traffic, commonly known as the Vienna Convention on Road Traffic, is an international treaty designed to facilitate international road traffic and to increase road safety by establishing standard traffic rules among the contracting parties. The convention was agreed upon at the United Nations Economic and Social Council’s Conference on Road Traffic (7 October – 8 November 1968) and concluded in Vienna on 8 November 1968. It came into force on 21 May 1977. The convention has been ratified by 74 countries, but those who have not ratified the convention may still be parties to the 1949 Convention on Road Traffic. This conference also produced the Convention on Road Signs and Signals.
Look at all the countries that are signed on. The overwhelming majority of the developed world, except for most notably the USA, Canada, Australia, and China.
My guess is because driving in general caught on early in the US and was very popular and so there was a need for our own standards early on.
This would be a fascinating article idea for the Jalopnik writers.