[cameronh90]

That's a problem with 4 way stops. Equal priority four way stops are just bad road design and should be replaced. Similarly your side street situation is why traffic lights were invented. Alternatively, you could give priority to the side street and force cars on the main road to yield.

That being said, I do think standardising some form of "go ahead" message is worth considering due to the prevalence of these poor road designs, particularly in the US. In the UK, officially, you're not supposed to make any form of signal. If you must, you are supposed to just stop, but that can be ambiguous, so most people flash their lights. However, that is unambiguously against the highway code. If it was possibly to explicitly state, through a light or some other kind of means, "I am yielding my priority", that could be unambiguously understood by both AI and humans alike...

That being said, these situations where you need to rely on messages are actually reasonably rare on the UK roads and usually as a result of roadworks messing something up.