Hmm, they're pretty synonymous, but I think you're noticing this slight, occasional difference in use: Lethal is active, deadly is passive. A thing can actively be lethal when used by you, but when it's something that happens to you, it's deadly. An accident is something that's considered to have happened to you, despite the fact that it's typically your fault to some extent.

I'm no linguistics expert but these are the definitions from Webster

lethal applies to something that is bound to cause death or exists for the destruction of life. lethal gas

deadly applies to an established or very likely cause of death. a deadly disease

They are synonyms and most people would probably use them interchangeably. I guess the biggest difference is lethal applies to something that is about to cause death, whereas deadly applies to death that has moreso already happened.

lethal weapons, deadly accident, etc ...

They're the same meaning just about exactly. Maybe lethal is a little more "fancy" if that makes sense. There's a lot of pairs of words in English like that, where one synonym came from old germanic/norse languages and the other came from old french/latin languages.

That's why wedding vows say "to have and to hold", for example. More educated people back in the day would use "have" (from habere in latin) and more common people would use "hold" (idk exactly from where but i assume old german or something.) When there was a wedding they wanted everyone to understand what was being said.

Thanks for all the replies. I had a very grim interpretation where the driver being an active part was removed. This happens a lot in German media where it is rarely a driver killing someone but instead someone "dying in traffic" - as if it's a higher power.

Glad it might just be interchangable or an alliteration...

@dmrzl @silence7 I think they're largely interchangeable, like "lethal crash" and "deadly crash". Perhaps the use of deadly here was for alliteration: deadly drivers.