That doesn't sound entirely true. I've only seen one-bulb versions for flashing yellow lights (which are sometimes used on crossings to indicate "pedestrians a

That doesn't sound entirely true. I've only seen one-bulb versions for flashing yellow lights (which are sometimes used on crossings to indicate "pedestrians are crossing" at difficult crossings, typically when more than two roads meet and the pedestrians could easily be overlooked). I've only seen two-light versions for pedestrians (i.e. red and green). I think I've seen other one and two bulb versions on non-public roads (e.g. industrial sites) but that's about it. I've never seen a version with four bulbs. And you make them sound more modular than they are: while the part containing the bulb seems to be generally modular the case containing these modules is generally fixed for a certain size. So adding or removing modules after they are deployed is not as straightforward (except maybe for temporary traffic lights). I've actually seen the auditory clue box (whatever it's called -- the ticking thing at crossings to help blind pedestrians cross safely) replace one of the bulbs (on three-bulb lights where there is a redundant second red bulb) instead of being added below the traffic light.