Why We Need a Fourth Traffic Light

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If there were a religion based on road markings, their Bible would be the text of the 1968 “Vienna Convention on Road Signs and Signals,” and their Holy Trinity would be the three illuminated components of the standard traffic light: the Red Light, the Yellow Light, and the Green Light — three separate entities, and yet still one whole, the traffic light itself. Well, now researchers at North Carolina State University have proposed a new fourth light for the standard traffic light — a white light, and its usage would be tied to automated vehicles.

Actually, the current announcement is actually an expansion of the original quartet of lights-proposal from February of last year, but this new one factors in pedestrian traffic as well. With 66% of drivers reporting they’re afraid of self-driving cars, it’s not unreasonable to think that one reason is a lack of understanding what an autonomous car might do. The fourth light is designed to help bridge this gap.

Think of it this way: You’re in a regular, non-self-driving car. You pull up to a traffic light at a typical four-way intersection hoping to turn right and there are three self-driving cars in front of you. To make things more complex, there’s also a guy pushing a stroller to your right hoping to cross the street. In a normal situation with no self-driving cars you’d know what’s likely to happen. But what about a situation where many of the cars can speak to one another? In theory, those connected cars could behave differently and more efficiently except… you aren’t a connected car and neither is the pedestrian.

Essentially, the new white light would be used in conjunction with 1. automated, self-driving vehicles with the ability to communicate with other automated vehicles and traffic signal control computers, and 2. human-driven vehicles with similar car-to-car and car-to-infrastructure communication abilities.

Here’s how Ali Hajbabaie, co-author of the paper and an associate professor of civil, construction and environmental engineering at NC State University, describes the concept:

“Our earlier work introduced the idea of a fourth traffic signal called a ‘white phase,’ which taps into the computing power of autonomous vehicles (AVs) in order to expedite traffic at intersections – but we had not yet incorporated what this concept would mean for pedestrians. We’ve now expanded our computational modeling to account for foot traffic, and the results are extremely promising for both pedestrians and vehicles.”

To understand what these papers propose, let’s look at what they mean by the “white phase,” which presupposes the existence of automated vehicles that are capable of communication with one another and traffic light signaling systems. For this to be effective, there would need to be a critical mass of these vehicles, along with human-driven vehicles able to connect to the same networks as the automated ones, and neither of these categories of vehicles are here yet.

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Here’s how the abstract to the original paper explains the “white phase,” which is not referring to that summer you swore to only wear alabaster or pearl:

This study presents a vehicle-level distributed coordination strategy to control a mixed traffic stream of connected automated vehicles (CAVs) and connected human-driven vehicles (CHVs) through signalized intersections. We use CAVs as mobile traffic controllers during a newly introduced “white phase”, during which CAVs will negotiate the right-of-way to lead a group of CHVs while CHVs must follow their immediate front vehicle. The white phase will not be activated under low CAV penetration rates, where vehicles must wait for green signals.

Essentially, it works like this: Connected automated vehicles are talking to one another and to traffic lights, and when they all compute an ideal, optimized traffic flow pattern, they can group together and take control of the traffic through an intersection by triggering the white light, which would tell human drivers to just follow the vehicle in front of them through the intersection, which can reduce traffic delays by a pretty wildly varying amount, from 3.2% to 94.06%.

The study also claims a 40.2% to 98.9% reduction in total delay compared to a “fully-actuated signal control obtained from a state-of-practice traffic signal optimization software.”

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So, what do we make of this fourth traffic light? Essentially, this light indicates that some interconnected group of automated cars is taking over traffic flow patterns at that intersection, and human-driven cars should follow.

According to Hajbabaie again,

“Our previous research found that the more AVs there are on the road, the more efficiently the traffic moves,” Hajbabaie says. “To be clear, this improves travel time, fuel efficiency and safety for all of the cars on the road – not just AVs.”

I have no doubt that the mathematical models used for this study do in fact show that traffic can flow better and easier with fleets of AVs evaluating things and talking to one another. But, the issue here is that in the actual, real, grimy world we live in, so far the companies working on automated vehicles do not appear to be focused on developing common vehicle-to-vehicle (V2V) communication standards necessary for this to work. This lack of progress on universal V2V has been noted before by far more reputable sources than myself.

For this fourth light to actually make sense, a lot has to happen. We need more automated vehicles, a lot more, and we need a car-to-car communication standard and much wider-spread car-to-traffic-light-systems, sort of similar to what Audi uses for their Traffic Light Information system.

I don’t think this white light will be added anytime soon, because for it to make any sense, we’d need large numbers of AVs and a comprehensive and widely accepted car-to-car communication system, and we’re just not there yet. It’s a good idea for finding a way to incorporate human-driven cars into the traffic flow optimizations of AVs in a relatively low-cost, well-understood way (people know to pay attention at traffic lights, and the only rule, follow the car ahead of you, is easy enough to convey) so in that sense I think the white light is a smart plan.

Oh, and if you don’t like a white light, the authors of the study are understanding:

“And, just to be clear, the color of the ‘white light’ doesn’t matter. What’s important is that there be a signal that is clearly identifiable by drivers.”

Yeah, it should be something other than white. How about ivory? Or eggshell?

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Mercedes Streeter

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104 thoughts on “Why We Need a Fourth Traffic Light”

  1. I know I’m really late to the discussion… username checks out, I guess.

    I encounter a lot of traffic lights as a pedestrian or cyclist. The experience is terrible with 3 colors plus arrows. Making a left turn at a busy intersection as a pedestrian or cyclist is an exercise in patience. Bike lanes don’t help.

    A traffic light controlled by automated machines doesn’t promise to make things any better. In fact, I suspect the opposite would happen.

    The fact that Jason and most of the commenters here seemed to miss the perspective of the non-driver is maybe not shocking, since this is a car-centric blog.

  4. There is so much low-hanging fruit in the traffic light area that could be picked without requiring a poorly-thought-out fourth light or widespread adoption of AVs (which it is not entirely clear will happen at scale in the foreseeable future).

    The other problem with any proposal to improve traffic light infrastructure is that most municipalities will not invest the money. My city can’t program our not-so-smart traffic lights correctly so I don’t have a lot of confidence in their ability to implement this.

  6. People still struggle with traffic circles and come to a full stop at flashing yellow lights won’t be able to figure this one out either.

      2. If there is a person crossing, you 100% are supposed to stop at a flashing yellow. (google RRFB). If it is a run of the mill intersection warning light or a full signal in flash mode, sure you don’t have to fully stop.

        1. If there’s a person crossing, you’re supposed to stop regardless, light or not.
          Same goes for a crosswalk here.

          The flashing yellow doesn’t really matter in that scenario. Pedestrians always have right of way here, and everywhere else I would imagine.

          1. Absolutely, the flashing light or even the signs or crosswalk paint don’t give pedestrians the right of way…because they already inherently have it with it just being an unsignalized intersection.

            I encourage everyone to look up “unmarked crosswalk” in their local and state traffic code. It will blow people’s minds. It is probably, along with going 5 mph over the limit, the most frequently and casually violated law in the country.

          2. Pedestrians do NOT always have the right of way. On a sidewalk, maybe, but they do not have the right of way on a roadway (often defined as a highway).

            A pedestrian has the right of way ONLY when in a marked crosswalk. If that intersection os controlled by a traffic signal, then the pedestrian has the right of way when they are in the marked cross walk AND ONLY during the tine the signal indicsted to “cross”!

            A pedestrian in any part of the roadway outside of a cross walk, or away from the intersection, is considered at fault if ever he/she gets hit by a car.

            I’ll quote Texas Transportation Code but similar laws apply elsewhere…

            Sec. 552.002. PEDESTRIAN RIGHT-OF-WAY IF CONTROL SIGNAL PRESENT. (a) A pedestrian control signal displaying “Walk,” “Don’t Walk,” or “Wait” applies to a pedestrian as provided by this section.

            (b) A pedestrian facing a “Walk” signal may proceed across a roadway in the direction of the signal, and the operator of a vehicle shall stop and yield the right-of-way to the pedestrian.

            (c) A pedestrian may not start to cross a roadway in the direction of a “Don’t Walk” signal or a “Wait” signal. A pedestrian who has partially crossed while the “Walk” signal is displayed shall proceed to a sidewalk or safety island while the “Don’t Walk” signal or “Wait” signal is displayed.

            Sec. 552.003. PEDESTRIAN RIGHT-OF-WAY AT CROSSWALK. (a) The operator of a vehicle shall stop and yield the right-of-way to a pedestrian crossing a roadway in a crosswalk if:

            (1) no traffic control signal is in place or in operation; and

            (2) the pedestrian is:

            (A) on the half of the roadway in which the vehicle is traveling; or

            (B) approaching so closely from the opposite half of the roadway as to be in danger.

            (b) Notwithstanding Subsection (a), a pedestrian may not suddenly leave a curb or other place of safety and proceed into a crosswalk in the path of a vehicle so close that it is impossible for the vehicle operator to stop and yield.

            Sec. 552.005. CROSSING AT POINT OTHER THAN CROSSWALK. (a) A pedestrian shall yield the right-of-way to a vehicle on the highway if crossing a roadway at a place:

            (1) other than in a marked crosswalk or in an unmarked crosswalk at an intersection; or

            (2) where a pedestrian tunnel or overhead pedestrian crossing has been provided.

            (b) Between adjacent intersections at which traffic control signals are in operation, a pedestrian may cross only in a marked crosswalk.

            (c) A pedestrian may cross a roadway intersection diagonally only if and in the manner authorized by a traffic control device.

      3. You can be required to come to a full stop at flashing yellow lights if they’re for a pedestrian crossing. Which is why I hate the way they implemented those. Flashing yellow does not mean “stop” it means proceed with caution, but they used it as a stop sign.

        1. Indeed, but depending on state law they may correspond with “yield to pedestrians” not stop. Same in practice though. The rapid flashing ones (RRFBs) are cost effective way to get most people to comply with the law. To go to a red light, it takes a much more expensive piece of equipment (a full signal or PHB).

    3. In my area there is an intersection that has a solid green arrow for right turns. The street the right turn merges with adds a lane at the intersection, hence the reason for the solid green arrow. Almost every time if there is traffic in the other lanes people come will come to a dead stop in a clear lane with that green arrow. One day I will post dashcam footage of the phenomenon to idiots in cars on Reddit.

    4. Absolutely this. First, the human driver needs to see and know all about the white light. So the message is: just follow along with the automated cars. Uhh.. for starters, which cars around me are automated? What a leap of faith that would feel like.

  7. As with most of these types of papers, simulations are done based on rule-following, best case scenario and simply forget that we are still dealing with humans.

        1. That’s just adding something to your vehicle to trigger the sensors already in the road. I think what the comment was about was the emergency vehicle triggers that can change lights in intersections such that there is a green in their direction. Not all intersections have the ability for emergency vehicles to change their signals, but enough do that it could make a difference for the incredibly selfish person.

          2. Didn’t flashing your headlights make it change at some point? (As long as there wasn’t a cop around to see it because they write tickets for that..)lmao

  10. I like the concept in theory, and I’m sure in the years between now and any adoption there will be further development, but I have one question;

    The design says that it’s up to human controlled cars to follow the car ahead, but what if they’re not going the same direction as the car ahead of them? What if one is crossing traffic and you’re intending on travelling straight? How would it judge this based on cars that aren’t integral to the system, because that would probably mean that car has to yeild, and now everyone behind them, including self driving cars that were probably anticipating free travel are now waiting for the primary light cycle anyway?

    1. You know those times you’re sitting at a red light but there are no cars crossing in those green light lanes? It’s stupid and wasteful right?
      That is one of the problems this tech could allegedly solve.

      Actually autonomous vehicles arent really needed.This could be done to any traffic lights,assuming the tech can be made reliable.Yes,that’s a big IF
      Personally i doubt it’s possible yet,but it’s nice to dream

      1. In a perfect world intersections would all be roundabouts full of patient, zipper merging, attentive drivers.

        Technology is going to solve the big ol’ problem of having to wait patiently at a red light for thirty seconds?

        There’s a larger systemic problem in your argument that you aren’t seeing clearly and that I can confidently ascertain you are a part of.

        What’s your fucking hurry?

        1. Waiting thirty seconds at a red light when there’s no traffic at all on green? WHY?? Why are you so determined to make everyone suffer needlessly? And grow some fucking manners

      2. We already have technology for this. Lane occupation sensors. Most intersections these days are equipped with them, where if they detect a car is approaching a red light but there is no traffic at the cross-wise direction, it will change the cross-wise direction to red and then will change the approaching or stopped vehicle’s light to green. Usually the delay’s only like two to three seconds.

        The downfall of lane occupation sensors is when some dumbass traffic engineer decides induced demand is a real viable practice and throws four lanes and two turning lanes at all four directions of an intersection.

  12. Every traffic light already has white lights. Dark orange at the top, light orange in the middle, white at the bottom. (left, middle, right, respectively, in them sideways states)

    Traffic lights only look “red”, “yellow”, and “green” in drawings, like the one in this article In the real world they don’t look anything like those colors. I have never in my life seen a green light that was actually green.

    Wait…that might be the deuteranomaly talking. OK, never mind.

    Please, don’t put another white light up there. It’s confusing enough as it is. Could I interest you in a lovely cobalt blue?

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