I don't think the parent was talking about traffic signals, just a side road with a give way/yield. If both roads are congested then "always let everybody in"

I don't think the parent was talking about traffic signals, just a side road with a give way/yield. If both roads are congested then "always let everybody in" and "never let anybody in" will both result in deadlock. Another example is the pedestrian crossing outside my house. There's no traffic lights. Usually cars will drive right past and you (as a pedestrian) have to wait for a gap in the traffic, but sometimes they'll be nice and stop (especially for children).