Study: Traffic signals need a fourth color

[wanderer2575]

As more self-driving cars enter roadways around the world, many aspects of driving could change forever.  More electric vehicles (EVs) and autonomous vehicles (AV) are already leading engineers to implement electric roadways and city planners to design dedicated lanes for AVs -- and now, the humble traffic light is next for a makeover.

For the dawning age of the self-driving car, transportation engineers from North Carolina State University are proposing the addition of a fourth "white light" whose function would be to alert humans to simply "follow the car in front of them."

That's because to leverage the full potential of autonomous cars means to also embrace their ability to act as a kind of hive mind.  Because they're essentially computers on wheels, these cars can network with both the traffic light and other nearby self-driving cars to help ease traffic delays and increase fuel (or electricity) efficiency.

Popular Mechanics, 10/28/2025

https://www.popularmechanics.com/technology/infrastructure/a69178503/fourth-color-traffic-light-white-study/

[NE2]

April Fools is that way ---->

[kurumi]

Stomatopods have been advocating for this for years (where "advocating" means punching a hole in the aquarium glass)

[kalvado]

We discussed it a few years ago.
https://www.aaroads.com/forum/index.php?topic=32879

[Max Rockatansky]

So the takeaway is that Popular Mechanics has now also reached the point at which they are recycling three year old stories. 

[vdeane]

What happens if the human driver needs to go a different place than the car in front of them?  If they're going straight and you need to turn left, are you then stuck not getting to your destination the way you want to?  And what happens if a human does something unexpected?

[Dirt Roads]

What happens if the human driver needs to go a different place than the car in front of them?  If they're going straight and you need to turn left, are you then stuck not getting to your destination the way you want to?  And what happens if a human does something unexpected?

My understanding was that all left turns were signalized as segregated onto separate turn lanes at any four-color traffic signal.  Therefore, if a non-autonomous vehicle (ergo, a plain ole' car or truck) is following an autonomous vehicle and the rattle-trap gets a "white ball" then they are getting a protected movement to proceed right behind the autonomous vehicle.  In almost all cases, right turns are already protected when the through lane gets a "green ball". 

But the "white ball" eliminated all of timing to transition between phasing when autonomous vehicles having conflicting movements are being interleaved in the approach to the signal. 



One of the issues we have in the Personal Rapid Transit (PRT) industry is when the interlocking (ergo, an mechanically-controlled intersection) is quite long, a PRT car with a malfunctioning propulsion system might take too long to the clear the intersection when the wayside control system has cleared the next PRT car to proceed on a conflicting route.  The safety measures we take to protect what railroaders call the "end-of-train" effectively adds back all of that transition timing between conflicting routes.  You can liken this to a fast wayside-autonomous Tesla trying to make a left turn behind the path of a slow wayside-autonomous semi-truck plodding through an uphill intersection at 10 MPH.  My analysis indicates that a four-color traffic signal controller should only "check-in" autonomous vehicles that are the roughly the same length and approaching the intersection above a minimum speed.  But I am known to be overly cautious in these calculations.

[wanderer2575]

So the takeaway is that Popular Mechanics has now also reached the point at which they are recycling three year old stories. 

A thousand pardons.  I didn't see the topic and didn't know the idea has been around awhile.

At least I didn't bump a thread that's been dormant for three years, right?

[vdeane]

In almost all cases, right turns are already protected when the through lane gets a "green ball". 

Not when pedestrians are involved they aren't.  And if that's using a white signal, then there's the potential for left turns to be an issue, too.

[Max Rockatansky]

So the takeaway is that Popular Mechanics has now also reached the point at which they are recycling three year old stories. 

A thousand pardons.  I didn't see the topic and didn't know the idea has been around awhile.

At least I didn't bump a thread that's been dormant for three years, right?

Further in your defense Popular Mechanics did change the publish date of the article.  The only way we would know is that an older thread to the same article link existed. 

Besides, it isn't like searching for special threads is easy nowadays. 

[Dirt Roads]

analysis indicates that a four-color traffic signal controller should only "check-in" autonomous vehicles that are the roughly the same length and approaching the intersection above a minimum speed. 

BTW, this presents a logic quandary for the four-color traffic signal engineer.  The railroad methodology for a train that doesn't "check in" properly (or with the proper credentials) is to prevent that train from entering the interlocking.  However, the air traffic control approach assumes that you can't stop the "offending" airplane and you protect the other airplanes from entering the trailing airspace.  In the slow-truck example above, since the "offending" might be running fully autonomous it probably makes sense to let it keep running through the intersection and slap the conflicting movement with a "red-ball" and make it wait until that portion of the intersection is cleared.  But like the railroad methodology, you could slap the slow-truck with a "red ball" and let the next vehicle through (which may not be the conflicting movement that we've been talking about).  It's not obvious which methodology results in improved throughput.

[Dirt Roads]

In almost all cases, right turns are already protected when the through lane gets a "green ball".

Not when pedestrians are involved they aren't.  And if that's using a white signal, then there's the potential for left turns to be an issue, too.

Here's where the driverless train industry deviates from the autonomous vehicle industry.  In most systems, it is imperative to eliminate hazards such as people, animals, large tree branches, and other potential obstructions from the driverless train environment.  Most systems require a full-shutdown just to let somebody work in the same tunnel with a driverless train.  Just sayin'.