Quick thoughts: Traffic Control Systems

Sean Wragg
Sean’s Blog
Published in
3 min readSep 18, 2016


I’m sure we’ve all experienced that moment in traffic: sitting in an intersection, waiting for the light to go green. Then, probably yelling at the immediate vehicle in front of you for sleeping through the light when it finally changes in your favor.

If you’ve experienced this more than you’d like, you’re in luck: MIT predicts that Red lights will probably become a thing of the past.

However, have you ever thought about how a traffic intersection works? It may be more complex than one would initially think.

Citybound SIM

Obligatory History

The world’s first, manually operated gas-lit traffic signal was short lived. Installed in London in December 1868, it exploded less than a month later, injuring its policeman operator. Traffic control started to seem necessary in the late 1890s and Earnest Sirrine from Chicago patented the first automated traffic control system in 1910. It used the words “STOP” and “PROCEED”, although neither word lit up.

Pasadena Traffic Signal 1928

Signal Rules

Alright, so let’s establish some ground rules for most basic Signal Lights — as they can vary throughout the world.


  • Vehicle must come to a complete stop
  • Vehicles can “turn right on red” but, them system doesn’t need to explicitly know about it.


  • Vehicle may stop if able to do so safely
  • Signal light will soon be Red


  • Vehicle may proceed if intersection is clear

Now obviously, this becomes more complicated once you introduce Turn Signals for traffic turning either left or right.

Traffic Control Approach

These days there’s generally a fewways to go about traffic control flow
(if we eliminate the possibility of vehicles communicating with one another):

Fixed time control

  • Lights are on hard timers between 30 and 120 seconds.

Dynamic Control

  • Lights change in favor of vehicles present using various means of detection

Coordinated Control

  • Attempting to give drivers a long string of green lights in succession

Adaptive Control

  • Automatically adjust light timing based on how long vehicles have been present

Each having their own advantages (and disadvantages). However, there’s also several other factors that most system additionally features

Emergency vehicles

  • Need the ability to stop all traffic at an intersection.

Non-motorized user detection

  • Pedestrians attempting to cross an intersection will affect which lanes can move at a given time.

Part-time operation

  • Disabling traffic lights at night or when traffic is low

Bus and Transportation Priority

  • Some traffic signals are activated to coincide with the arrival of a bus or tram.

Railroad preemption

  • Some traffic signals are activated to coincide with the approach of a train

Manual override

  • Most systems can be manually operated using a local junction box

Originally published at seanwragg.com on September 18, 2016.



Sean Wragg
Sean’s Blog

Writing about code, comics, and fhqwhgads!