"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 (

(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.