It is a total, sorry, mess.
Have some fun with this site:
Goodyear’s guide to driving in summer in Europe2
Whether you are a local or a tourist taking on a scenic summer driving holiday, it can be challenging to navigate the different road regulations across Europe. Although the summer season generally brings more favourable driving conditions, driving in...
or this screenshot:
A full list of differences in traffic regulations, traffic signs, speed limits and penalties probably runs for many, many pages…
Most shockingly, when you move countries in Europe (as I did twice), they just hand you over a new, national driver’s licence- no questions asked and, what is actually quite scary, no information given.