wunder beta🚦 How Traffic Lights Work
A police poster from 1868, a box on the corner that overrules its own computer, and the wire under the tar that decides whether you get a green.
What you’ll learn
- Whose turn is it?Explain what a signal is scheduling, and what each indication does and does not promise.A signalized intersection is a timetable for movements that cannot share the space, and the handover between them is a calculated interval, not a guess.
- Inside the grey boxDescribe the cabinet hardware that runs an intersection and how it senses demand.A controller applies a written plan, an independent monitor can overrule it, and buried loops turn arriving vehicles into calls that stretch or skip a green.
- Everyone else at the intersectionTrace how pedestrians, coordinated corridors and emergency vehicles claim time from the plan.Crossing time, green waves and preemption are the same argument about who gets the next few seconds, bounded by clearance rules nobody is allowed to skip.
Questions this course answers
What does a steady green ball actually permit?
The federal manual grants the movement but keeps the duty to yield. Only a green arrow marks a protected movement the timetable has cleared of conflicts.
Why is a yellow interval usually between three and six seconds rather than one fixed number?
The kinematic equation uses about one second of reaction time and roughly ten feet per second squared of deceleration, so the interval scales with approach speed.
How does an inductive loop know a car is sitting on it?
Metal near the loop opposes its magnetic field, so the inductance drops, the oscillator speeds up, and a threshold crossing is reported as a vehicle.
Match each part of the intersection to what it does.
Phases are the units of the schedule, calls are its inputs, offsets link one intersection to the next, and the monitor is the independent check on all of it.
What does a flashing upraised hand with a countdown mean?
The clearance time is sized for somebody who left the curb at the last instant of WALK, walking at three and a half feet per second.
A fire engine triggers preemption on the cross street. What can the signal change, and what must it still do?
Preemption reorders the schedule but cannot delete the transition that gets vehicles already inside the intersection safely out of it.
Grounded in trusted sources
- Federal Highway Administration, Manual on Uniform Traffic Control Devices, 11th Edition (December 2023), Part 4 - Highway Traffic Signals: https://mutcd.fhwa.dot.gov/pdfs/11th_Edition/part4.pdf
- Federal Highway Administration, Traffic Signal Timing Manual (FHWA-HOP-08-024): https://ops.fhwa.dot.gov/publications/fhwahop08024/fhwa_hop_08_024.pdf
- Federal Highway Administration, Traffic Detector Handbook, Third Edition, Volume I (FHWA-HRT-06-108): https://www.fhwa.dot.gov/publications/research/operations/its/06108/
- Transportation Research Board, NCHRP Report 731: Guidelines for Timing Yellow and Red Intervals at Signalized Intersections (NCHRP Project 03-95): https://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP03-95_FR.pdf
- Federal Highway Administration, Emergency Vehicle Preemption (FHWA-HOP-24-019): https://ops.fhwa.dot.gov/publications/fhwahop24019/fhwahop24019.pdf
- Michael Luo, "For Exercise in New York Futility, Push Button", The New York Times, 27 February 2004: https://www.nytimes.com/2004/02/27/nyregion/for-exercise-in-new-york-futility-push-button.html
- Metropolitan Police, "Police Notice: Street Crossing Signals, Bridge Street, New Palace Yard" (1868), via Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Traffic_Lights_1868.jpg
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