Actually, if as you approach a traffic signal, you cede control of the car to the signal, you can avoid red lights altogether as the smart signal shuffles the cars through without any of them hitting each other, handing off control back to the car after it passes through the intersection. A similar technique could be used on on-ramps.
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).
