(Radio:) "Talk about those evil ramp lights."
"Ramp lights. Yeah that's another genius invention that they created."
He's really good at playing a grump on the radio.
(Radio:) "Hey we need to back up traffic down the freeway ramp
all the way into the surface streets..."
He does make a good point. After a long day, all you want to do is get home.
You wait for several stoplights just to get to the freeway --
and then, on the on-ramp, there's another stoplight!
Seemingly for no reason!
Traffic engineers claim that by slowing you down just a little bit here on the ramp,
it'll speed you up quite a bit on the freeway over there.
(Bendelhoum:) It goes much better much smoother and we move more cars.
Is the wait really worth it?
(Title graphic)
So are traffic engineers just putting in another traffic signal for fun?
Well, no. Traffic signals cost a lot of money --
and each one they install slightly elevates the crash risk. Somebody
might not be paying attention and will slam into a stopped car.
No, this traffic signal does something really neat.
And to find out how it works, I decided to ask somebody.
(Bendelhoum:) We try to move people as fast as we can, without breaking
the the flow of traffic on the mainline.
Mohammed Bendelhoum uses ramp meters to smooth out traffic flow on some of California's freeways.
At the red light on the city street below the freeway on-ramp, our cars stack up.
You and I and Hector and Phil and Karen (who is actually really nice) --
all get a green light and turn left together.
Our group of cars is called a "platoon."
(random military noises)
We race up onto the freeway as a big bunch. And when the freeway is busy,
our multi-car intrusion messes up the flow of traffic on the freeway.
A ramp meter makes each one of us take turns --
and when we enter the freeway one car at a time the freeway flow is less likely to get messed up.
But to really dig deep into why ramp meters are really cool, you need to go --
TO TRAFFIC SCHOOL!
Time for Traffic Flow Fundamentals 101!
(Don't worry, it's not as scary as it sounds.)
This free-flowing road has no stoplights or stop signs.
Let's imagine each car spaces themselves one mile apart from the next car.
That's density.
Density is a frozen snapshot of a moving freeway.
Look at how crowded each lane is at that exact moment.
Then there's speed.
Which you know very well. No need to explain that one.
When we multiply DENSITY with SPEED, the unit "miles" cancels out.
We're left with a flow rate 60 cars per hour.
More cars on the road and that density number goes up.
A bigger density number on the left --
means a bigger flow rate on the right.
The freeway can move more cars. Flow rate is the name of the game.
If we keep that number higher than the demand for the freeway
and you'll never see another traffic jam again.
Take note how the less crowded freeway lanes actually move more cars than the crowded ones.
Now, if you're an electrical engineer you must be thinking about this equation:
AMPS x VOLTS = WATTS
And say, okay, well amps are kind of like the density of
the cars on the road. And they have a limit.
Volts are like the speed limit. Which is limitless -- with enough horsepower!
So, if you want to move twice as many cars on a totally full road --
All you need to do is just drive twice as fast, right?
(sassy graphic)
Ohm's Law doesn't really work in traffic engineering. Instinctively you do know why.
It's because as we drive faster, we tend to spread out more.
It's the old "two-second" following rule from driver's ed.
Being glued to somebody's bumper is fine when you're going 10 MPH, like I am right now.
But as I speed up to 60 MPH, I'm going to need hundreds of feet to be able to travel safely.
Electrons don't do that. They're little tailgaters who stay right
on each other's bumper -- no matter how many volts get thrown at them.
So getting more flow rate means you can't just UP
density or UP speed. Because each of those two cancel each other out.
It's about finding a sweet spot for both.
Engineers graph all of this with speed (Y-axis) -- faster goes up
against density (X-axis) -- more crowded to the right.
And it kind of makes sense. Can you drive 70 MPH on a nearly empty freeway? Sure!
How about driving 70 MPH on a kind-of-full freeway? Well, yeah.
There's still cushion for everyone to cruise along and merge in and to change lanes.
But what about a freeway which is tight? One with barely enough room
for your car plus the two seconds of following distance you need?
Well, that's when freeway flow risks falling off a traffic cliff.
(ouch)
Okay, finally. Here's where this all ties into ramp meters --
and a little bit into lakes.
If you have a freeway that's flowing but traffic is heavy, it's really dense --
A shock can throw that freeway over the traffic cliff too early.
You might be able to squeeze another 10 or 20% potential out of a freeway.
But out in the wild, shocks rob a freeway of its full potential.
We need to find a way to get rid of them.
It could be something as simple as a broken down car on the right shoulder.
Or it could be somebody making a simple lane change
Or it could be traffic merging in as a big platoon (hint hint...)
(Bendelhoum:) The cars on the freeway will start to break,
because these cars are trying to merge in. They start braking. It's like a ripple effect.
Think of it almost like an accordion.
When the instrument is wide open there's room for the bellows to move back and forth.
But when the instrument is nearly closed, it becomes really hard to play.
When i'm driving down a low density freeway
there's room for me to brake and not slow down the car behind me.
But on a high-density freeway, everybody cascades in a sea of brake lights.
And those big platoons of entering cars are one of the biggest culprits.
If you can break the platoons up into little pieces
(small plop sound, no louder than a mouse)
It's not as splashy when you do it that way.
I wish I knew how to skip stones.
That would really drive (<-- PUN) the point home.
(Bendelhoum:) We break that platoon and make it go
like one car at the time. It can merge into the flow of the mainline of the freeway.
And it works like magic.
Compare these two rush hour simulations,
which a fan of the show cooked up for me in some professional modeling software.
The line at the meter looks insane.
But watch very quickly how much better the freeway moves because of it.
Even with that extra wait at the meter,
your overall travel time would be significantly shorter.
(Bendelhoum:) We use the congestion on the mainline of the freeway to trigger
the ramp meter cycling. All our ramp meters operate in the traffic responsive mode, 24/7.
So here in California, the ramp meters kick on at about 1,600 cars per hour.
That's about two-thirds before it hits the maximum flow.
And the computers can kick on all by themselves. You don't have to
have an engineer come out and manually switch it on and off.
They don't even have to switch it on
and off at a control center. The computer does all the thinking.
(Bendelhoum:) The congestion on the mainline lane dictates when to start metering.
It can turn on any time of day 24/7.
So if you have the Lakers losing to the Jazz after double overtime...
(NBA Live 95:) The Jazz defeat the Los Angeles Lakers 63-18
...at two o'clock in the morning,
and Staples Center is just dumping car after car after car onto the 110 freeway,
those ramp meters can kick on -- even in the middle of the night.
The same thing happens during morning and afternoon rush hour.
(Bendelhoum:) They have a flashing beacon at the top that flashes when the meters are cycling.
Because normally a freeway ramp is free-flowing. You don't stop.
And so a warning sign gets a driver's attention,
Thinking, oh yeah, I gotta pay attention. There's a signal down there I've got to stop for.
So how do engineers safely switch a freeway on-ramp from free-flowing?
(Bendelhoum:) To meter, it starts on a green ball -- usually like three to five seconds.
Then it goes to yellow and then it goes to red. Each green light allows one car to
enter the freeway at a time. And then after that it just goes from green to red green
red red green right. And then it goes from one lane to the other.
Breaking up splashy platoons into little pebbles.
The release rate is how often the meter lets a car through.
(Bendelhoum:) When the density of traffic and the volume of traffic reaches a certain threshold,
the controller looks at those values and changes the rate automatically without our intervention.
And as easily as the ramp meters switch on,
they can also just as easily switch off when they're no longer needed.
Which i learned the hard way!
That is twice now! Everything was perfect,
ready to film, and the meter turned off -- right as I was ready to record.
(Bendelhoum:) Each time there is a project,
we put in the ramp metering system. We have to trench for conduits.
They buried a loop of wire -- a loop detector -- which can
sense cars driving over it at the top of the ramp. That lets the
computer know how many cars to expect -- how many are coming down to the light.
There's another loop of wire right at the light. It reminds the computer,
"hey there's somebody there who really wants to actually get on the freeway--"
and there's another loop of wire just beyond the meter,
to let the computer know that somebody really did go when the light turned green.
And the creme de la creme -- there's a whole bunch of loop detectors down
on the freeway that lets the computer know the current conditions: how crowded the freeway is.
Still, no matter how well
a ramp meter is configured -- freeways do eventually fill up.
We pass maximum density and everybody brakes to a stop.
(Bendelhoum:) Sometimes you get to the freeway and you are in a parking lot, you know!
And if that's the case, you may be wondering:
Well, if it's all an exercise in futility, why are we metering in the first place?
It doesn't look like it's working but it's actually making the rush hour slightly shorter.
So let's go back to this graph. I'm going to redraw it a little
differently and replace speed with flow rate -- which again is SPEED x DENSITY = FLOW RATE.
How many cars the freeway is moving and it shifts the graph to look like this.
The density starts with just a few cars per mile --
so even though they're flying along at full speed,
the flow is low because there just aren't very many cars.
On the other end, we have tons of cars -- bumpers nearly touching bumpers. But the
flow rate is also low because those cars are traveling slowly or stopped.
What we want is the peak and we want it to be as high as possible:
dense and fast.
When a shock comes early, the flow peaks early. And the peak is far too low.
Meters kicking on pushes the curve back up much
more quickly than if we just let traffic take its natural course.
Now I'm REALLY going to mess with the graph and move density to the y-axis on
the left and put time on the x-axis on the bottom. 7:00 A.M. on the left and 9:00 A.M. on the right.
This dotted line is jam density.
That's where the freeway clogs up because there's too many cars.
In the wild, this is what the graph would look like.
These big clumps are traffic jams -- morning crowding.
Meters shorten those jams. They start later. They clear sooner.
And those little tiny slivers on each side
mean more time for good flow. And good flow moves more cars.
A ramp meter lets engineers be good stewards of tax dollars and really
stretch the capacity we have make sure we get every dollars worth of freeway we already have.
And that can help us delay or potentially cancel widening projects.
So about 20 years ago, Minneapolis decided to try switching off all 400 of their ramp meters --
just for two months, to see what difference the meters really made.
It's a fun story and it's one I'll be covering in an upcoming video.
(Garrett Schreiner, MNDOT:) I'm assuming it's the people that want us to meter more are the
people that are on the [freeway] main line -- driving by the ramps where people are coming on.
So keep an eye on the description for when that video comes out.
So do ramp meters to work?
Yes
Minneapolis saw their freeways lose 10 percent of capacity with the meters turned off.
Travel time took 20 minutes longer.
And side-swipe crashes doubled.
So they turned the meters back on. And that emboldened
other cities to roll out their own meters.
(Ad-lib) Okay, this thing recording?
So check this out: This is a ramp meter on a freeway.
Putting a meter on a ramp between two freeways is a little bit tricky because
you definitely do not want cars backing up onto the 405 freeway.
But it isn't much of a problem here. There's this long
continuous freeway connector ramp -- probably over half a mile long.
And of course -- it's 2020!
And i drove all the way out here and the meter's turned off.
But you've seen it and that's where it would be turned on if it WAS turned on.
Who would've ever thought I'd be complaining about there being not enough traffic in LA?
Your in-laws -- your mother-in-law is moving in with you and they need
a place to stay in your house. But you live in a little tiny house it's barely big enough for you.
So what you do is decide to spend $90,000 adding a guest bedroom to your house.
It works.
It's nice.
It's preferable.
But it costs a lot of money.
A ramp meter is more like making your kids share a bedroom. They're not gonna like it. But if you
stretch your money just a little bit further, you can postpone that big home renovation.
Tweet me: @roadguyrob