Award-winning traffic engineer Paul Basha analyzes: Is lengthening the yellow light a good idea?

By Paul Basha, Summit Land Management | Rose Law Group Reporter

Since the traffic engineer world is highly dependent on human behavior, yes-and-no.

In the 27 January 2023, Friday, Arizona Republic, an article describes the City of Phoenix in-process study of determining the answer to the question.  The article reports that in metropolitan Phoenix, from 2014 to 2020, there were 113 deaths and 9,300 injuries directly related to people driving through yellow lights, when they should have stopped.

So, would be a good idea to have a good idea to prevent people from being killed or injured by other people failing to stop on a yellow light.

The City of Phoenix Street Transportation Director, Kini Knudson, has adopted the exact correct response to this question. Let’s see.  The City of Phoenix has lengthened the clearance interval at a dozen unnamed intersections, and will study the data for a minimum of 12 weeks.

As the article explains the “clearance interval” is the time of the yellow light plus the time of the all-red light. Also explained in the article is that the all-red time is the time between the end of one’s street red light and the beginning of the intersecting street’s green light.  Traffic engineers call this the clearance interval because its purpose is to ensure that all drivers that very recently saw a green light become a yellow light are clear of the intersection before the drivers on the intersecting street see their red light become a green light.  After the yellow light for one street, before the green light for the intersecting street, all drivers at the intersection see a red light.  Traffic engineers, highly intelligent and clever people that we are, call this the “all-red time”.

Once upon a time, prior to the early 1960’s, there were no all-red times. It was a good and simple world then.  Drivers saw a yellow light, and stopped.  The drivers on the intersecting street saw a green light simultaneously, and went. Difficult to imagine now. Sometime in the past six decades, people decided they needed to hurry, and felt superior to artificial intelligence dictating that they stop their vehicle so other people could safely go.  People decided that other people could wait for their vehicle to travel through the intersection.  And usually, they did.  Yes, back then, polite and respectful people outnumbered impolite and disrespectful people.  The poor driving behavior of rude and contemptuous people seldom had consequences.

Over time, the polite people learned from the impolite people.  Increasingly people began to drive through intersections on yellow lights, rather than stopping. Providing some people were polite, nothing bad happened.  Then some of the polite drivers waiting at a red light that became green, decided they were too important to wait and look for oncoming vehicles, and they drove through the intersection, before the first disrespectful person cleared the intersection. Soon, many collisions occurred, and many people were seriously injured, and some people died.

So, one or a group of traffic engineers had a good idea to prevent people from being killed or injured.  They decided to insert a short red light after the yellow light before the other street green light.  All was good and simple again. Even impolite and disrespectful people became polite and respectful.

Then drivers learned that the red lights were longer than they were before the all-red was implemented.  Because they were more important than other people, their time was more valuable than other people’s time.  And they could not wait for the longer red lights.  They soon discovered that the longer red lights gave them more time to clear the intersection before the intersecting street drivers received a green light.  So, they became more bold and disrespectful in driving through yellow and all-red lights.  And, usually, there were no consequences for their poor and unsafe behavior.  Though soon, many collisions occurred, and many people were seriously injured, and some people died.

So, some people think it a good idea to lengthen the yellow plus all-red light so that everyone stops before other people go.

When I was Scottsdale Traffic Engineer, in the late 1980’s, our maximum all-red-light time was 2.50 seconds. Our Traffic Engineering office collaborated with the Police Department.  I knew one police sergeant well, in part because we both attended Cornado High School football games.  He became increasingly disappointed that he had to take seriously injured people out of seriously damaged cars and put them in ambulances very often at one particular intersection.  It was an intersection near our respective homes that both of us drove through often, with our young children.  He asked me to increase the all-red time to 3.50 seconds.  One second could prevent injuries and deaths, we hoped.  I reluctantly agreed to increase the all-red-time.  For several weeks, the intersection operated much more safely.  We were both happy that he did not have to help seriously injured people out of their damaged cars and into ambulances.  The world was good and simple again.

Then it wasn’t.  Far too soon, perhaps three months after the all-red-time changed, the police sergeant once again was often called to serious-injury-collisions at that intersection again.  Drivers had learned bad behavior, and drove through the intersection on the longer red light.  He suggested that we return the all-red-time to 2.50 seconds.

The length of the yellow plus all-red time, the clearance interval so that all cars stop before other cars go, is determined by the speed limit on the streets.  If people are driving at the posted speed limit, and the green light becomes yellow, then all-red; they will have time to stop.  If they are driving over the posted speed limit, and the light changes from green to yellow to all-red; they will not have time to stop.

Some say that traffic engineers should increase the length of the clearance interval because many people exceed the posted speed limit.  A longer yellow plus all-red time makes the red lights longer.  So, all people have longer red lights and longer waits and longer delays, because some people are too impolite and disrespectful to drive the speed limit.  And, all-too-quickly, the rude and contemptuous people learn that yellow plus all-red time is longer, so they have more time for their misdeeds.

In the mid-1990’s, while still Scottsdale’s Traffic Engineer; I met a lovely lady who two years or so prior, had been in her car waiting at a red light at an intersection.  When the light changed to green, she drove.  On the intersecting street, a man was late for his golf game, so he did not stop when the light facing him changed from green to yellow to red, dan the cars collided.  He was very late for his golf game that day, though he was able to continue to play golf for the next six months, while the lady spent those six months in what was then called Scottsdale Memorial Hospital.  She was told that she would never walk again.  Well, she was a strong-willed, determined, ambitious, and otherwise healthy person; and after six months, she walked out of Scottsdale Memorial Hospital; with no assistance; very slowly and very awkwardly and very painfully.  I met her when she testified at the Scottsdale City Council asking that Scottsdale implement photographic enforcement.  Which they did by a 4 to 3 vote.

Correct; 113 deaths and 9,300 injuries, from a single cause, in seven years in a metropolitan area of 4.7 million people; demands a good idea to prevent it.

Drive the speed limit.  Stop on yellow lights.

Curious about something traffic? Call or e-mail Paul at (480) 505-3931 and

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February 2023