What is congestion?
What we're going to cover in
the next few minutes
is exactly what this title suggests.
And that might seem
like a funny question.
Everybody knows what congestion is.
But what I really want to convey
is that there are two ways of looking at
what happens when a road becomes
congested and understanding
these two ways of looking at it.
They're not mutually exclusive.
Both are correct in their own way.
But understanding these two perspectives,
I think, is going to help us move forward
and think about ways
to solve congestion and understand
why some common solutions
don't work
and also understand down the road
why some solutions do.
So let's get right to it.
So the first perspective
I want to talk about
is for for lack of a better way
of putting it,
the engineering perspective.
Like what actually happens mechanically
when a road breaks down
and it's level of service
and traffic starts to slow down?
And so let me let me start to give you
some terms here.
There's basically
two categories
that we sometimes talk about
when we talk about congestion.
And the first one
is called recurring congestion.
And that's just what it sounds like.
It's the type of congestion
that predictably and reliably
shows up day in and day out.
And so the obvious
example of this is sort of peak
period commuter travel, right?
That, you know, the four
or five freeway here in L.A. at 8 a.m..
The second one is called
nonrecurring congestion.
And that's sort of the unexpected
or less regular events
that create bottlenecks.
Perhaps the most common of these
is sort of traffic incidents and crashes,
whether a road is closed
for construction
or two vehicles
hit each other and slow everything down.
Three closers for special events,
weather related incidents.
These aren't
perfectly sort of distinct categories.
You know,
one of the most obvious ways
they interact
is that the more recurring congestion
you have, the greater
the chance that two vehicles
will crash into each other.
Sometimes you do
see someone
trying to assign,
you know,
40% of the delay in Saint Louis
is to recurring congestion
and 60% to non-recurring.
I don't think those
are terribly accurate,
but it's good to understand
these two categories. Right.
And so just to really drive
this point home.
Here's a classic scene
of a rush hour freeway
that's recurring congestion. Right?
If all these folks
are on their way to work
or things like that,
that's probably going to happen
every day.
Non-recurring congestion again
can take different forms.
Sometimes it's something really unusual.
A road is closed for a parade,
and then as a result,
all the roads around it
are much more congested
than they would normally be.
That's nonrecurring congestion.
Another example of nonrecurring
congestion would be something
that's not quite
so unpredictable and irregular.
For instance, here in Los Angeles,
when the Dodgers
play a home game on Sunset
Boulevard near
their stadium, gets much more congested
than it would be otherwise. Right.
And so that's also non
reocurring congestion.
So we have these two categories.
Let me give you a couple more key terms
for us to work through.
So the first one is speed.
And I think probably all of us
are comfortable with what speed is.
But this is basically the distance
that a vehicle can cover
and a unit of time.
So 55 miles per hour.
The next one is flow.
This is the number of vehicles
that pass a reference point
per unit of time,
and it gets measured as sort of vehicles
per hour or vehicles per 5 minutes.
This is also sometimes called volume,
to give you a sense of how
how flow can be measured.
Just imagine standing at a certain point
along the side of a road
and counting cars as they go by. Right.
And if five vehicles go by per 5 minutes,
then that's that's
your measure of flow or volume.
The next term is called headway.
And the headway is
just the time
that elapses between vehicles
at a certain point.
And so it's basically the the inverse
or the reverse of flow.
And then lastly is density, which is the
the vehicles per unit of roadway
and we measure
this often
as the vehicles per lane mile of road.
And a lane
mile is exactly what it sounds like.
It's a mile of road in one lane.
And we measure
roadway capacity
and lane miles
to account for the fact that some roads
are wider than others.
Right.
It's better measurement than just the
the total linear
distance of roads,
which we call centerline miles.
Understanding these things
helps us better
understand
a very important point
about the way a roadway can function,
which is that
you can get high or low flow
in very different
sets of traffic conditions.
And let me kind of illustrate
that and just kind of illustrate
some of these terms.
So here you have
what looks like
a kind of rural highway
that is not super heavily trafficked.
And so this would be characterized
by high speeds, low flow, large headways
and low density.
And you can see all that in the well,
you can't see the speed in the picture,
but you can imagine
that these cars are moving pretty fast.
But you can see the low flow.
There are too many cars there. Right.
So if you were to stand
along the side of this road,
you just wouldn't see too many cars
go by you.
Part of the reason for that
is that there's a lot of space
between the cars.
That is, in fact,
what's letting them go fast. Right.
And so that's the large headway.
And then because there aren't
too many cars on the road,
the density is low.
In contrast,
here's a congested urban road,
and this is characterized by low speed.
Right.
You can see these cars aren't going
very fast, high density.
The cars are very close
to one another,
filling up the road, low flow
because the cars aren't going very fast,
not too many of them
are going to go past
a particular point
in any given amount of time
and then low headway
as the cars are very close to
one another.
There's very little space between them.
And I want to emphasize a point here,
which is that both of these pictures,
right, this one
and that one have low flow. Right.
Which means that low flow
can occur both at very high speeds
when there's lots of space between cars
and very low speeds.
Right.
When cars are very close to each other,
but not moving.
And so if you were to plot speed and flow
the way that it
kind of manifests
in the typical American roadway
or roadway around the world,
for that matter,
what you would get
is what traffic engineers
call the backward bending curve,
which is what you're looking at
right now.
This is the plot of speed versus flow.
And what you can see is that on the far
left side of the graph, that flow is low.
When speeds are very low,
right down around in the lower
left hand
corner, down around ten miles
an hour or so,
but also at speeds are very high,
right up near 80 miles an hour.
And again, just to drive this point home,
the reason that happens
is that
when cars are very close together,
they just can't go fast.
And when they can't go fast,
they can't move
past a point very quickly.
And then, conversely,
for cars to be able to go
very fast,
80 miles an hour, there simply
can't be that many of.
So a road
that's letting someone travel
80 miles
an hour is a road
that can't be carrying
that many vehicles.
So that slide you just looked
at was just kind of a stylized depiction
of this figure,
which I'll come back to in a little bit,
is an actual representation
of data
collected by sensors
on the Ten Freeway in Los Angeles.
And it's not nearly as clean
as the sort of stylized picture
I just showed you,
but it does clearly show
that there is a backward bending curve.
Right.
And again,
I'm going to describe
this more in a few minutes.
But what you do
see is that flow is quite low
when speeds are quite high at 4 a.m.
and flow is quite low
when speeds are quite low at 9 a.m..
Again, I'm going to come back to this.
And so what's going on right
with this in mind,
if all that makes sense,
we can now start to think about you're
driving along
a close to rush hour in an urban area.
What happens
when when traffic congestion
sort of envelops you
and what basically happens
is that while you're driving
in free flow,
you know, if you're not in rush hour,
what we say
is that a well-functioning road balances
high speed and high flow.
Right.
And what we mean by that
is that when the road's working
the way we want it to,
you have a lot of vehicles
all moving
quickly,
maximizing the capacity of the road
that the most vehicles
are passing over a given space
on the road
in a given amount of time as possible.
And so when this occurs,
what we say
is that the density of vehicles
on the road is at its optimal point.
Now, when traffic arises,
all of that starts to fall apart
and it falls apart
because the density actually rises
above that optimal point.
You get a few too many cars
on the roadway, right?
And so
that's what causes delay
to begin to accumulate.
How does that happen?
It's the following.
So again,
in a stylized, perfectly operating road,
drivers are basically going
to stay at precisely
the minimum safe distance from each other
and travel to constant speed.
In other words,
if we were all sort of perfect drivers
that when
when you were driving down the freeway
and another car
started to drive on from an on ramp,
you would perfectly calculate
how far away
you needed to be from that car
to maintain safety
and just adjust your own driving
in a way that allowed that to happen
and it allowed other vehicles
to get on the road with minimal friction.
Now, of course, in the real world
that's not how driving happens,
that basically all of us react
to the uncertainty
of a new driver on the road
in different ways.
Some of us are going to when we see
a new car
entering the
the highway from an on ramp,
some of us will speed up
to get ahead of it.
Some of us will stand on our brakes
to sort of really slow down
and let that new car in.
Some of us will switch lanes. Right.
And when we switch lanes,
the person
driving along behind us in that lane,
they have to react as well. Right.
And what will they do?
They might switch lanes themselves.
They might stand on their brakes.
They might try to move into the lane
that we were just in.
All of this creates
a little bit of friction.
All of it
slows down
other cars by just a tiny amount.
But because there are a lot of cars,
those tiny amounts of delay start
to add up.
Right.
That basically
because we all react differently to risk
some of us
leaving room
between cars, other of us
tapping the brakes.
And all of this uncertainty increases.
Then collectively,
all the delay
that we're imposing on each other
starts to accumulate
in what's
almost like a shock wave, right?
A wave of delay
that starts to roll backward
down the road
because as one driver slows the cars
in close
proximity, behind it
are going to slow down as well.
And what happens to you
when you're sort of driving along
and suddenly
every
all the traffic is slowing down
is that you're basically driving
into one of these waves of delay. Right.
And then it might ease up for a while
and then you drive into another one.
And this is the phenomenon of stop
and go traffic. Right?
So so essentially what's happening
is that because we all react differently
to the uncertainty
created by other drivers,
we create some friction
and that friction turns into delay.
And that delay adds up
and all of that starts
because enough vehicles are on the road
that it makes us feel like
there's some risks to being on the road.
Right.
And cities
to really think quickly about this.
None of this really happens
if you're driving at 4 a.m.
and there's nobody on the road
but you, and then you notice
one more car getting on the road.
Right?
You don't feel like
you have to react to that other car.
But at 8 a.m.,
when the road is already quite crowded
and you see some other car
trying to merge onto it,
now you feel some risk
and that risk causes that reaction
and that's where you start
to get the congestion.
Now, just as an aside, occasionally
you hear about how self-driving cars
could reduce congestion.
The logic behind that
and just for the record,
this solution,
if it ever comes,
is probably pretty far off.
But the logic behind
that is that
autonomous vehicles
that can talk to each other
are going to sort of
mathematically compute
risk in a much more accurate
and consistent way than humans.
And so as autonomous vehicles
flow onto a road
that's already crowded
with autonomous vehicles,
the idea behind this
is that they can all sort of calibrate
and communicate perfectly and maintain
the correct distances
between each other, anticipate each
other's reactions
right,
and therefore allow
more vehicles onto the road
with less friction
and therefore less delay.
Again, I don't tell you that
because I think that's going to happen
any time soon.
I think we're a long way from sort of
like perfectly communicating
autonomous vehicles.
It's just another way of thinking
about what's actually going on
when a highway full of cars driven
by humans gets into congestion.
It's the fact that we all react to risk
and we all react differently, right?
That creates this sort of friction
between vehicles.
So the engineering perspective, in short,
is this idea that
either road hits a density of traffic,
that kind of tips
it into this delay accumulating reaction.
Right. Okay.
So now we come back to this slide
that I posted earlier or this image
that I posted on an earlier slide
of the I-10 freeway in Los Angeles.
And what I want to illustrate with
this is something I've hinted at before,
but it is an important point,
which is that congestion
isn't a linear phenomenon.
And go back to what I said
earlier
about the idea
of driving on a road for AM
and a car gets on the road,
you know, alongside you,
probably that car will get on the road
and you'll be on the road
and neither of you
will have to slow down.
Right.
There's plenty of room
and then another car
could get on the road
and not none of the three cars
would have to slow down. Right.
But at a certain point,
another car is going to get on the road
and then all of you are going to have
to slow down a little bit. Right.
And so that's what
I mean by non linearity,
which is that up to a certain point,
a road can absorb new cars with no delay
at all.
Once it hits that point,
delay really starts to accumulate.
And you see this
very clearly in this
in these data from the I10,
which is that at 4 a.m.,
the I10 is pretty empty.
What you see is a sort of low flow
and high speed
that the average speeds
about 68 miles an hour.
You're moving about 133
vehicles per 5 minutes
and it stays that way.
If you follow the graph from left
to right for quite some time
and you can just imagine
that this is
still the early hours of the morning
and more vehicles
are getting on the road,
but not enough to slow it down
until if you
look over to the right at 5:35 a.m.,
you see that
the flow is quite large, right?
It's a 607 vehicles per hour
and the speed is declined by not
but not by a tremendous amount.
The that you still kind of
have the vehicles moving in free flow.
They're going about 47 miles an hour.
But then after that, the flow
and the density hit a point
where everything starts to fall apart
and you can see the line
dart, you see that backward
bending curve come back
until at 9 a.m.,
sort of the height of Los Angeles
rush hour,
both the speed
and flow have fallen that you're moving
493 vehicles
per 5 minutes and only going 35
miles per hour.
Right.
That after those 550 or so vehicles,
each additional vehicle has large
negative effects on speed.
And I think this is something
that's sometimes hard to grasp.
Right, which is that
what that means, among other things,
is that a really congested
road is actually,
despite the fact that it looks
very crowded,
is actually moving in fewer vehicles.
Right.
And this is an image of the
the ah, the S.R.
91 expressway in Orange County.
And what you see
and I'll come back to this in part two,
what you see it
right in front of you is the tolled part
of that expressway where you have to pay
to get on the road.
And on the right,
you see free lanes, right?
And so there's two tolled lanes
and four free lanes.
And just in that
in a cross
sectional sort of
the static image
that we see is just a snapshot.
It really
looks like there's many more cars
going by
on those four lanes
than on those two lanes.
But if you measure the flow
of the two lanes
and the four lanes,
what you actually see
is that they're almost equal.
And the reason for that
is that on the lanes, on the laugh,
the cars are moving so much faster.
If you took this picture
5 minutes later,
many cars would have passed by
in the lanes
that are right in the foreground,
the two lanes that are told,
while many of the same cars
would be in the image on the right.
Right.
The fact of that high density,
the fact of that low speed means
the flow is low.
So and this is
sometimes hard to intuit in many ways.
But if you've ever watched
the old TV show The Three Stooges,
there was a famous gag
in the Three Stooges
called The Doorway Gag,
which is that the Three Stooges
would all get excited about something,
and they try to run
through a door all at once,
and as a result, they'd get stuck. Right.
And what basically happens in traffic
congestion is very similar to
what happens in the doorway gag. Right.
Which is that precisely
because so many people
want to get on the road at once,
they all slow each other down
and they
they can't move through as quickly.
And what happens in the
in the Three Stooges
is that the MOE,
who's the leader of the Stooges, tells
them, tells everyone to spread out,
and then when they spread out,
they can all walk through the doorway
very easily and get through.
And this is
if you think about the Three Stooges
as a mini traffic problem, right?
You can start to see how this happens,
which is to say
that if you looked
at the took
a snapshot of the Three Stooges
while they were in the doorway,
you would say that the
the volume was pretty high.
There's three people in the door,
but the flow is pretty low
because they're stuck.
Right?
Whereas if you let them each walk
through individually,
you would see at any given moment
the number of people was low,
only one person in the door,
but the flow is high
because they're proceeding through
very quickly.
The same thing happens
on the roadway, right?
Is another way of just emphasizing
this is not a linear phenomenon.
Okay.
So that's the engineering perspective.
That's how I think
a civil engineer
might look at the problem
of traffic congestion.
What we're going to do in
the next
chapter
is look at the economic perspective,
and I hope you'll join me for that.