There's a lot of research on possibilities here! eHMIs (external human-machine interfaces) are the term.

https://www.theturnsignalblog.com/blog/ehmi/

https://www.mdpi.com/1424-8220/21/9/2912

It's unclear that there's a consensus on something that reliably works across cultural contexts, though - which, to be fair, is also a problem with human-human interactions!