In Germany, vehicles have emissions ratings, with a corresponding windshield sticker. https://www.germanemissionssticker.com/ Vehicles below a certain threshold are banned from "low emission zones" defined by local governments; most are densely built inner city areas.

I don't think they make much of a difference reducing traffic, since the requirements for a rating that will let you drive everywhere are really easy to fulfil. But I suppose they trim the worst of the worst.