Brown Mazda, looking to avoid the light— gets the right turn on the lane,
blows past a white Chevy on the yellow light and cuts back over!
TOUCHDOWN!!!
Yellow lights are out to get you!
Don't run the yellow light, they say.
Except for the times when it's safer to run the yellow light.
Because sometimes— stopping... is far more dangerous.
Let's start off with something really exciting.
This is a traffic signal cabinet, and inside there's wires, and power supplies and most
importantly, the traffic signal computer.
A little bit of brain power that tells green lights to be green and red lights to be red.
And most importantly, yellow lights to be yellow.
Imagine if traffic signals went straight from green to red.
Yellow is obviously really important.
It gives us time to decide if it's safer to keep going or to stop.
That's not easy.
As you point out, sometimes there's this "awkward zone."
You don't know if you should be mashing the brakes or pressing the gas.
If you have enough space to comfortably and safely come to a stop, that's what you should
do.
If you can't stop, then you should have enough time to be able to clear the yellow light
before it turns red.
In this case, this car didn't.
And you could see it in this diagram.
The green arrow shows how far you'll continue to travel if you maintain speed.
Not enough.
The red line shows the braking distance.
We can get stopped safely in time.
So that's what we really should do.
Group two, are people who should run it.
The circumstances are basically flipped now.
Our stopping distance is too long, but we have enough time to get into the intersection
or maybe even beyond before the light turns red.
So let's go.
These are the times when continuing to go is safer—
because you might be able to stop, but a big heavy vehicle behind you might not be able
to.
Mass and speed play a huge role.
They're going to end up in the intersection anyway, so they might as well maintain speed
and go through into the intersection— and they should have adequate time to get
there before the light turns red.
When we put the drone up in the air, we can watch drivers make this decision in real time
on a real road.
California Highway 79— near the dump!
[Love Boat parody:] ♫ "The Dump!
Exciting and new!" ♫
Anyway, there's this invisible line, right.
Drivers beyond the line do not have space to stop safely
but drivers behind it do.
And as we watch this, you can see this real
line of traffic categorizing themselves appropriately.
Those that should go continue to go.
Those that should stop, stop.
Both of those first two groups you intuitively know, it's the third group we regular people
never talk about.
But you know what it is!
You know that awkward zone you were telling me about?
It's called a Dilemma Zone.
And it's an awful situation to be in.
There are two kinds of Dilemma Zones: The second type involves human psychology.
It's not easy for engineers to fix.
So we'll dive into that in the next video.
A type-one Dilemma Zone happens with yellow lights are too short.
It creates this overlap zone where, for some of the cars, they can't go or stop.
No matter which option you choose, you're getting a red light ticket.
Speed and mass make it worse, such as cruising downhill at highway speed in a big rig!
The driver saw the light turn yellow, didn't know what to do and finally panicked, slammed
the brakes.
That's an expensive stop.
So what happened?
Well, let's rewind the tape— right up to the moment when the light first
turns yellow— and freeze!
The driver saw [the light].
And there's a mathematical calculation of how much distance it'll take to stop.
Initially, he's okay, but we're not calculators.
It takes time for the brain to figure this out.
So as his brain reacts, the truck happily keeps cruising along at 50, gobbling up the
precious few feet he's got left.
It's now or never...
BRAKE!
What about going through the yellow instead?
That's what makes it a dilemma zone.
A big truck might only be going 50— but the yellow set for 65.
Guess who's running the red light?
A fresh red and big rig?
Several studies show short yellow lights cause more red light running
In turn, when engineers make yellow lights appropriately,
long intersections become safer and better to use.
So why don't we just double the length of yellow lights—
make these things 8 or 10 seconds long?
That would work, right?
Well, it's kind of like hand soap.
If you don't use enough, you'll have trouble getting your hands clean.
But if you use too much, it just makes everything messier and worse.
Intersections must be safe and efficient.
There's only so much time on the clock.
And the more of it you give to yellow, the less there is for green.
Extra yellow doesn't mean it's extra safe.
In 1980, the Federal Highway Administration found long yellow lights confuse drivers.
The researchers say excessively long yellow lights, "definitely are hazardous."
And there's something just satisfying about choosing to stop—
and watching the light turned red right as your bumper comes to the white line.
But if you stop and you see there's two more seconds of yellow, you feel like you're leaving
money on the table.
Next time, you're more likely to read it.
So how long is an *appropriately-set* yellow interval?
You've probably heard the old saying: one second of yellow light time for every 10 miles
an hour speed limit.
But that's actually not quite true.
That rule of thumb takes into account speed.
But there's also if we're going uphill or downhill, how quickly the vehicle can stop,
how long it takes for us to react to the yellow light—
and other factors which make engineering clarity a little bit foggy.
In fact, there's actually no national standard for setting a yellow light timing.
But it's not that engineers don't have help.
Since the 1920s, when an engineer first installed a yellow light, they have been debating how
long those lights should be ever since.
By 1985, they tried to vote on a unified standard, but everybody voted it down.
So in recent years, they just sent out a survey and asked engineers, what do you actually do?
Two hundred engineers responded with a quarter saying they set a uniform time.
All the stoplights have the yellow the same like no matter what.
Another 18 percent use a table of values that
base the length of the yellow on the speed of the street.
Another 18 percent said "other,"
and the remainder, thankfully, use an equation.
This is what it looks like.
Scary looking, but not really.
Let's walk through it.
That alphabet soup of letters on the left side of the equal sign.
All that's saying is: "How many seconds long is the yellow light?"
That second term is the perception reaction time.
When the light turns yellow, your brain needs time to figure out.
"Oh yeah, that's a yellow light."
You need time to make a decision of what you're going to do.
And you need time to move your leg to the brake pedal.
If you're deciding to stop research shows typically that's one second.
So that's what most engineers use.
Now we move to the big ugly fraction, but the top part's easy.
It's just speed — miles per hour.
Except we don't care about hours.
Nobody has an hour of yellow light time.
We have seconds.
So let's change that to feet per second by multiplying this.
Dilemma zones are really a problem on faster streets.
If you have to really jam on the brakes for a yellow at thirty miles an hour, it's not
nearly as big of a deal as having to do it on, say, a road that 60.
This assumes we're going the speed limit, but we don't.
Researchers looked at 800 intersections.
The speed drivers actually go is often ten miles per hour over the posted speed limit.
So they recommend we bump this up by seven miles per hour—
to make the yellow light a little bit longer to reflect that faster speed.
Not every city does this.
So if you'd like to zip along with the rest of us, it may partially explain why yellow
lights feel short.
On the denominator of our equation: this number, which represents deceleration.
How quickly you can get a car stopped, added to gravity—
multiplied by whether you're going uphill or downhill — the grade.
When you're going uphill, Mother Nature helps slow you down more quickly, but when you're
going downhill, she's not helping much.
The denominator will become smaller, which means you'll have a longer yellow light.
[TV:] "Drivers got red light tickets they never should have been issued."
A TV station in Portland, Oregon, decided to play with the formula themselves.
[TV:] "You'll see what KOIN 6 discovered.
"For a long time it was a lot shorter.
Just four seconds."
Like the TV station, you could also find an intersection with a short yellow light and
do the same calculation to see if it's close.
So let's try it.
Beaverton, west of downtown Portland, just off the OR-217
at Hall Boulevard and Scholls Ferry.
So what do we need to figure this out?
Well, we need to know the speed limit.
Google Maps, jump down, get a street view of it—
and there's our speed limit: 40.
Okay, braking is something engineers have researched and 10 is the number that we go with.
And grade, it looks pretty flat to me— so I'm just going to put in zero.
So let's calculate all these numbers together, make sure you do order of operations where
you do the multiplication first— and then you add it together.
And of course, we get four seconds, which is actually what the bad time was that the
news was complaining about.
We just got the same wrong answer.
[TV:] "The city kept its mouth shut."
Hold on!
That the city probably made an honest mistake, because we just made the same mistake.
What are we doing wrong?
That's right.
We put in the speed limit and we need to put in the speed people actually drive.
So let's change that to 47, run the numbers on through and—
[happy noises] There you go.
Four point five seconds.
[Celebration:] "The greatest formula of all time!"
Well, not quite.
An electrical engineer in Oregon spotted a problem with the formula after his wife got
a red light ticket.
[TV; Mats Jarlstrom:] "I'm here before you for the eighth time."
The formula assumes we're continuing at the same speed—
that adjusted speed limit.
That doesn't work when we're slowing down for a turn to turn right or left.
Nobody makes a right turn at forty seven miles per hour.
At least they shouldn't.
Honestly, it's probably more like 15 miles per hour.
It's a real mess because you have completely different dilemma zones for each direction.
Fixing left turns could be a little bit easier if they have their own protected arrow because
you can set its own special yellow time.
But you can't fix right turns.
It uses the same yellow light is going straight.
So the speed we plug into the formula isn't just 47 miles per hour.
The Oregon electrical engineer argued
we should also take into account the change of speed as you approach that turn.
In this case, subtracting 47 from 15 to represent slowing down to make the right turn—
and then plugging in 15 to represent continuing to make that turn.
Other values remain the same.
Calculating in the turning vehicle results in a much longer duration of a yellow change interval,
which is the entire point.
[TV, Mats Jarlstrom:] "That's where we have the civil rights violation."
[TV, Mayor of Beaverton:] "Can you wrap up, please, Mats?"
Mats Jarlstrom's story probably warrants its own video. If I ever get the opportunity to talk to him, I probably will make one. But the short version: He's an electrical engineer.
He won't let it go because he's an engineer.
So he keeps persisting at the city level and eventually the state level.
Oregon's licensing board fined him $500 for practicing civil engineering without a license.
He had to go to court to fight—
[music sting] for his right—
[parody of "Fight For Your Right to Party"] ♫ "TO TURN RIGHT!"♫
To advocate for safe traffic signals.
He argued anybody should be able to do that, not just licensed civil engineers.
He won the case— but the biggest victory of all, he got his
formula in the hands of the Institute of Transportation Engineers.
And after a long, careful review— in March of 2020, they published this article,
which basically says, "you know what, this guy might be on to something."
One persistent electrical engineer in Oregon may be responsible for the biggest change
to the yellow light formula in half a century.
If there's any takeaway from this video, it's that yellow lights are complicated.
And as much as we looked at here, we haven't even begun to scratch the surface of the amount
of research and potential research that we could do into yellow lights.
In the next video, we'll take a look at Dilemma Zones and how they affect our brains.
[Engineer:] "This car thought it could stop the trucks said, oh, no, I'm going way too
fast for this."
And how technology can work with us to make traffic signals and intersections safer.
[Captions made possible by Patreon contributors.]
[Thank you.]