
Phantom traffic jams, and why lane expansion isn’t really successful - ryan_j_naughton
https://www.cnet.com/roadshow/news/traffic-talk-ford-vanderbilt-2018/
======
gok
> Latent demand is the idea that if you widen a highway to reduce its traffic
> density, people who would otherwise have stayed on surface streets will now
> fill the newly available space on the highway, negating the benefit of
> having more lanes.

...if the only benefit of building more lanes was reducing mean transit time
at rush hour. You could also just build a one lane road and have the toll so
high that only a single driver could afford to use it; that would be optimally
“beneficial” in this model.

~~~
SerLava
Thank you. I am mystified by the apparently widespread mental framework that
roads don't actually do anything other than attract masses of people in cars,
inexplicably like insects toward bright light.

~~~
gascan
It's true that roads convey people & permit economic activity.

But that road down the way that is so congested right now? If we expanded it,
and it could convey more cars at the same level of congestion- _where did they
come from?_

The latent demand represents potential traffic that either:

1) is doing just fine taking a different route right now

2) isn't particularly economically valuable, i.e. not worth waiting through
traffic for

3) things like future sprawl enabled by wider roads today

In order, #1 is not particularly valuable to reroute, it was doing ok. #2 is
not very valuable either, though perhaps a little. #3 creates new demand, but
the only thing we get in return is lower density.

So I guess in sum the point is that all the most economically valuable travel
is already happening, right now, and by widening the road we add only the
travel that has such marginal economic value it's not worth doing today. So
the more we widen the road, the less & less economic value we get in return.

~~~
SerLava
>2) isn't particularly economically valuable, i.e. not worth waiting through
traffic for

With schedules and business operating hours, many economic activities are
impossible due to hard time constraints.

I don't see how your argument would be less applicable to a city with two dirt
roads.

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1123581321
I can’t speak to the aggregate, but going from two to three lanes in each
direction on I-90 in northern IL has cut travel times for us by about 8%
despite increased total traffic. This is because trucks passing now leave the
left lane open and because the speed limit was increased 5MPH.

~~~
repsilat
Also: induced demand is a _good_ thing. Even if travel time were the same with
three lanes instead of two, that's 50% more people moving down that road.
That's a quantifiable social benefit.

~~~
sjwright
> That's a quantifiable social benefit.

Only if you know how the occupants in these additional vehicles were
transiting previously. If they were previously carpooling, or if they moved
away from a good (but slightly less time efficient) public transport option,
then the social benefit would be somewhere between minimal and nil.

~~~
chii
But presumably the new occupants compared their existing old travel method,
and deemed the new one better. If the old method had some "social benefits",
it must mean that the old method's social benefit came at a cost in benefit to
the occupants. Otherwise, they would not have chosen the new method!

~~~
sjwright
Just because an individual makes a particular choice when given the
opportunity, doesn't mean it must be a social benefit.

~~~
sjwright
I love how I was voted down for that comment, one which ought to be obvious on
its face. There are plenty of choices people make that don't benefit society
as a whole. That isn't controversial.

~~~
sjwright
Rather than vote me down, perhaps you could reply instead? Use your words.

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martyvis
I just got a new Subaru Outback with their Eyesight Adaptive Cruise Control.
I've gotta say it is fantastic and just the sort of fairly simple and safe
technology that could ease traffic jams. Certainly outside the city where I
live, the way it nicely follows cars at a safe distance seems to make a lot of
sense. As soon as the lead car moves away, it neatly accelerates to the preset
speed and smoothly decelerates on the engine braking (or even real braking if
needed) when it sees the gap closing. It's very obvious on my long commute
that probably only 30% of cars seem to be using just regular cruise control,
based on how their speed varies on the clear open road.

~~~
stephengillie
Adaptive Cruise is life-changing. I couldn't go back to normal cruise.

A major downside to Adaptive Cruise is when another driver uses your gap to
change into your lane, and your car slams on its breaks because suddenly their
car is just inches away from yours.

Recklessly aggressive drivers are learning they can take abuse these safety
features in the cars of other drivers, making roadways potentially more
unsafe.

~~~
Already__Taken
What are you saying, you want it to crash into them for you? That's a major
downside?

~~~
zyberzero
I'm not parent, but just a note from me: I drove a brand new Volvo XC40 last
week that had adaptive cruise control. If someone just cut in to the front of
me the car slammed on its brakes even if it wasn't necessary. The car could
just as easily slow down gradually until necessary space was free in front of
the car (as I usually do when I don't have adaptive cruise control).

~~~
vinceguidry
Try Mazda, they're much better in that regard.

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blasdel
Bill Beatty's comprehensive explanation from 20 years ago is still the best:
[http://trafficwaves.org/](http://trafficwaves.org/)

~~~
peterwwillis
All he's doing here is obviating braking. In theory it could make for slightly
reduced trip time as some stop-and-go's have more inefficiency than just
moving slowly. He's reducing the pressure of the flow. But he's doing nothing
about the actual causes of the slowdowns, which is on-ramps, off-ramps and
rubbernecking. So while you reduce stop-and-go, you're still traveling at
maximum capacity, just like in stop-and-go's. A kink in a hose doesn't go away
if you reduce the water pressure.

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toast0
Lane expansion isn't likely to eliminate rush hour, but it will increase the
number of vehicles served, and can also reduce the duration of peak
congestion. As vehicle populaton grows, congestion will grow as well, however.

A recession is even better than construction at improving traffic, however.

~~~
sliken
The article has a lame explanation of latent demand. Sure some switch from
surface streets to the highway. But people also decide to make more trips,
commute further, or use public transportation less. So the result is after an
adjustment period of about one year there's almost precisely as much traffic
per lane as before.

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Nerdfest
If a lot more people drove manual transmissions, or at least learned that you
can slow down by just raising the accelerator a little rather than hitting the
brakes, things would be a lot better. It really doesn't seem like a hard thing
to learn.

~~~
sliken
I believe it's the reaction time lag, not using the brakes instead of the
throttle to slow down. Basically people suck at staying in the ideal position
half way between the car in front in the car in back. Because of that they
brake hard as they come close to the car in front. The next car has to brake
harder... till you end up with cars stopping. So it's the human reaction time
combined with cars accelerate (generally) slower than they brake.

~~~
rplst8
This. There are lots of studies that have pointed to slow re-starters as the
prime culprit for traffic. And it's only gotten worse with cellphones because
now there is something to longer distract people when the come to a stop.

~~~
lloeki
Slow-restarters _and_ sudden late-brakers. This is really all about an
oscillating wave being carried vs mean flow speed. The best part when you get
the hang of it is that in many situations when you can anticipate the "slow-
restart/sudden late-brake" of a dozen or so cars before you (and a couple
behind you), you can act as a dampener as you strive to keep near the mean
flow speed, and delightfully see the phantom jam downright _evaporate_ in your
rear view mirror.

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sliken
My favorite related quote is that adding lanes to decrease traffic is like
letting your belt out to lose weight.

For more info see:
[https://en.wikipedia.org/wiki/Induced_demand](https://en.wikipedia.org/wiki/Induced_demand)

~~~
ars
But you are ignoring the human and economic benefits of increased travel.

A better example would be someone pulling their belt so tight they can barely
move, and then letting it out.

~~~
SerLava
But we let the belt out all that way, and the belt still touches their waist!

~~~
ars
Then there is more demand. Keep going. There is nothing that has unlimited
demand.

You can't reduce demand by constraining supply, all you are doing is making
people unhappy.

If your policies are very free, you'll find people will figure out how make
things more efficient. For example inexpensive delivery will become possible
if there was enough space for traffic.

I know this because I've lived in dense cities (no delivery service - it's
simply too hard to find parking, and thus too expensive) and wide urban areas
(every store offers delivery because it's cheap and easy).

And that's _exactly_ the opposite of what everyone thinks will happen - they
want cars off the road, and replaced with delivery, without actually making
delivery practical (i.e. inexpensive).

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bayfullofrays
Limiting the use and possession of private cars would go a long way into
reducing environmental impact and income inequality.

~~~
magduf
That'd be great, but no one wants to actually build any workable alternatives,
at least in the US. A lot of people would probably be happy to give up their
cars (or at least keep them parked until they need them for weekend trips) if
they had good public transit available to use instead. But US cities are not
doing a very good job with that.

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badrabbit
I wonder if toll payment based on amount of time spent in a lane would improve
things? Right/exit lane would be free and incrementally increase up to the
high cost left lane. The lane stripes would have connected rfid sensors to
measure amount of timr spent in each lane.

Might work because less lane switching means less slow downs to adjust to new
cars entering a lane. And people who don't absolutely need to will remain on
right lanes while leaving the left lane for those committed to the lane or for
those using it momentarily to pass other cars. Heck,you can even change lane
pricing dynamically based on traffic and time of day.

~~~
sjwright
This would almost certainly cause extreme congestion on the exit lanes and as
a result a huge speed differential between lanes.

Moving to the faster lanes would be dangerous. Cars merging back into the
slower lanes to exit would cause permanent jams in the second lane at every
turn-off. Nobody would benefit in the end and the road would be horribly
unsafe.

~~~
badrabbit
That's why I suggested dynamic pricing and charging based on time spent. The
right most lanes would congest but they would also have exiting cars.

Second from right could cost a bit more than free (1cent per mile for
example). It would be an incentive to use it for passing but also to avoid
right lane congestion for a negligible fee. Third and fourth from right can
cost $0.20 and $0.40 per mile which discourages long term use but also allows
those in a hurry who can afford the fees to leave the highway faster and allow
for cars in right lanes to use less congested left most lanes to pass slow
drivers and lane exit/entrance in their lanes.

Think of it from a network traffic CoS and QoS point of view (also,sorry for
the late response,hope you see this)

~~~
sjwright
Again, a nice idea in theory but it will just concentrate congestion towards
the right, which would exacerbate the speed differential between lanes and
make the road much less safe.

It would exacerbate economic disparities, forcing poorer people—who are
already chronically time-poor as well as money-poor—to spend more time in
their car for the convenience of rich people. Rich people already have
innumerable advantages in our society, we don't need to go around looking for
more to give them.

(As a comparatively rich person, I appreciate living in a society surrounded
by people that are—in the main—well educated, healthy and not overly stressed,
and I happily pay a lot in taxes to help maintain that. Which is why the USA
holds zero appeal to me; you couldn't pay me to live there.)

~~~
badrabbit
Overall,there would be less lane changing and cars would leave the freeway
faster. The pricing can be adjustes or subsidized for low income drivers.

If you pay a toll anyways,this is an opportunity to save money and use the
tollway,where as you would normally take the slow and long unpaid route. This
would actually make toll roads accessible to the poor. No matter how I look at
it,if you add lanes,they should be paid for so that it would be more
affordable abd practical and actually incentivize drivers to cause less jams.

Also,traffic jams are not exclusive to the USA. Maybe it should be modeled in
a more politically convenient way? How about fuel tax deductions? You get n
deductions where you lose more deduction points by using left lanes? That way
there is no added cost to the driver,the tax payers as a whole pay for it.

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alphabettsy
I find that on my commute most of the time when one lane is moving slow it’s
because the vehicle at the front of a long line of cars, usually the left two
lanes, in traveling 5-10 mph slower than other traffic. Speed differential and
the use of brakes seems to cause traffic jams, so having vehicles all
traveling the same speed or at least moving out of the way if they’d prefer
not to go with the flow would seem to solve these issues.

~~~
bryanlarsen
"A long line of cars ... traveling 5-10 mph slower"

In other words you're saying that they're closely packed. If the distance
between cars is reduced by 50% and their speed has reduced 10-15%, the
_throughput_ of that section of road has _increased_ substantially.

As you said, it's speed differential and the use of brakes that causes traffic
jams. By that reasoning, the left lane should go the same speed as the right.
That would also discourage the use of lane changes which is generally what
causes people to use their brakes on the freeway...

And remember the square law of braking. Because the safe following distance
increases so much when you increase speed, faster speeds on the freeway
decrease throughput rather than increase it.

~~~
Sohcahtoa82
> And remember the square law of braking. Because the safe following distance
> increases so much when you increase speed, faster speeds on the freeway
> decrease throughput rather than increase it.

Got an article to explain this? It seems counter-intuitive.

Following distances should be based on time, not physical distance. It
shouldn't matter if cars are going 30 mph or 300 mph. If everyone maintains a
3 second following distance, there should still be a throughput of 20 cars per
minute.

~~~
bryanlarsen
At 10mph, safe following distance is 1.8 seconds. At 60mph, safe following
distance is 3.6 seconds. At 90mph, safe following distance is 4.4 seconds.

[http://www.adventures-in-
education.com/math/StoppingDistance...](http://www.adventures-in-
education.com/math/StoppingDistance.htm)

[https://en.wikipedia.org/wiki/Braking_distance](https://en.wikipedia.org/wiki/Braking_distance)

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andrewla
These problems only occur when the load get close to the effective maximum
load. It's easy to look for blame in driver behavior, but I think misleading
-- it's like taking that first crystal to form in 0 degree water, and blaming
it for ice formation. You may be able to reduce crystallization (and thus
sneak in a couple more Joules) by distilling the water (adaptive cruise
control) or cleaning your glassware (on-ramp metering or "do not change lanes"
signs), but really you're just fighting the statistics of phase changes.

Note that I don't have a ton of supporting evidence for this sort of thing.
One imaginary project that I would love to see if I had the time or the
patience for deep research is to just gather a data set and make it public of
anonymized traffic behavior -- over a large stretch of highway, have periodic
cameras that track every car by license plate (if possible) or car shape,
aggregate and remove the identifying data, and output a data set that allows
you to slice it by time or by moving segment to directly observe these things.

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hedora
I was hoping for more detail. The argument against adding more lanes is that
making the freeway faster than surface streets causes people to prefer the
freeway to surface streets.

Tying lane count back to fuel economy, average door to door time, and number
of vehicles served would be much more productive.

Similarly, the article recommends hov meters on freeway entrance ramps, but
doesn’t mention that they create jams sbout a half mile after the entrance.
Again, some sort of metric involving “how many people”, “how long”, and “how
much gas” would go a long way.

Fun fact about the Bay Area: my car gets about 75% of epa freeway mileage
driving from SJ to Santa Cruz (over a mountain), but only 50% from San Jose to
Mountain View (in traffic). This is if I try my best to conserve fuel in the
traffic case. If I drive aggressively, the gap widens.

So, reducing conjestion could substantially increase the miles per gallon of
the existing silicon valley fleet of cars.

Instead, the “environmentally friendly” nimbys keep voting in people that pay
for traffic quiescence “upgrades” that further snarl traffic and increase the
local carbon footprint.

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StanislavPetrov
Here in New York I would attribute traffic jams in equal parts to the endless
construction, massive congestion, and a huge number of terrible and/or elderly
drivers. All it takes is a couple of slow/bad drivers to clog up the
expressway for miles, even if the congestion is low and there is no
construction.

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a3n
If another freeway lane attracts drivers up from the surface streets, then
that must be good for the surface streets. I assume the new drivers must have
determined that that's a more efficient way to get from A to B.

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JTbane
Better idea- expand the lanes but give them to a light rail system instead.

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dwd
One solution is to restrict lane changes.

All it takes to create a phantom traffic jam is someone to change lanes where
the cars behind need to slow or brake to accommodate them.

