Ah, that's lovely. Very polite!
Yes, we went through the same thing with traffic lights, and he learned the names of all the different cars and had to name every one we passed/that passed us. Skoda, Ford, Audi OH A MINI!! WHEY HEY! BMW, volkswagon...
He still interrupts himself to shout 'MINI' and he's 4!
I think they're just making sense of the world around them and looking for patterns. There's a very clear pattern to traffic lights and they're bright and obvious so it's no wonder they're fascinated by them.
No particular hints to break the obsession. Just wanted to say I've been through it too.