Morgan Traffic Signal

Find the three settings in the drawings. Two are the obvious stop and go; the third raises both arms at once and is the actual subject of the patent.

Draw a crossroads and move two counters through it, switching instantly from one direction to the other. The collision is unavoidable — the junction is never empty at the moment of handover.

Time = junction width divided by vehicle speed. A slow vehicle already committed needs the whole crossing. This number is why the interval cannot be arbitrarily short.

Mount a vertical square post on a stable base. Everything pivots from it, so it must not rack when the arms swing.

Cut two arms lettered STOP and GO on each face. A signal legible from one approach only is worse than none.

Pivot one arm to face each roadway, mounted 90° apart so each governs its own direction independently.

A worm drive gives large reduction and will not back-drive — the arms cannot be blown or knocked out of position, which is a safety property, not a convenience.

Profile a disc so one turn drives: north-south go, all-stop, east-west go, all-stop. The cam IS the timing logic.

Shape the cam so the sequence cannot skip from one green to the other. If clearance can be bypassed by cranking fast, you have rebuilt the two-state signal.

Trip a striker off the cam just ahead of each transition. Morgan's patent includes an audible warning because a driver looking elsewhere still gets the message.

Provide a position that leaves all arms half-mast permanently — the equivalent of a modern all-way flashing red for quiet hours.

Crank slowly through a complete revolution. At no point should two conflicting GO faces be showing — check every intermediate angle, not just the detents.

Move vehicles through under your own signal. Introduce a slow one that enters just before the change and confirm the clearance interval gets it out.

Recut or block the cam so all-stop lasts half as long, and repeat the test. Watching the conflict reappear is the point of building it.

The patent. Garrett A. Morgan filed on 27 February 1922 and was granted US 1,475,024, "Traffic Signal", on 20 November 1923. Morgan, the son of formerly enslaved parents, was a Cleveland businessman and inventor who also devised the safety hood of 1914 — the breathing device he used personally in the 1916 Cleveland waterworks tunnel rescue. He is said to have decided on the traffic signal after seeing a serious collision between a car and a horse-drawn carriage at a junction.

What is actually claimed — and what is not. Morgan did not invent the traffic light. Electric signals existed before him; a gas-lit signal stood outside the Houses of Parliament in 1868, and Cleveland itself had an electric red-green signal from 1914. What Morgan patented is the T-shaped hand-cranked mast whose distinguishing feature is a THIRD position raising both arms simultaneously, halting all traffic so the intersection can clear. Getting this distinction right matters: the claim as often repeated is too broad and the real one is more interesting.

Why an all-stop interval is not optional. A junction has physical extent, so a vehicle that has legally entered needs time to leave. Switch instantly and you create a window in which one driver has right of way and another is lawfully still inside the box. Every modern signal encodes the same answer — the amber change interval plus the all-red clearance interval — and those durations are computed from junction width, approach speed and reaction time, exactly the quantities you measured in step 3. Morgan's mechanism made clearance a mechanical certainty rather than an operator's judgement.

What happened to it. Morgan sold the rights to General Electric, reportedly for 40,000 dollars. The hand-cranked mast was superseded within a decade by automatic electric signals, and the modern amber phase descends from other lineages as well as his. What survived is the logic: three states, not two, and a mandatory interval belonging to no one. The worm drive is worth a second look too — choosing a non-back-drivable gear so the display cannot be knocked out of position is a small piece of safety engineering done properly.