
Traffic flow measured on different 4-way junctions [video] - ltomic
https://www.youtube.com/watch?v=yITr127KZtQ
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mcv
This is from the game Cities: Skylines, which is basically the modern version
of the old classic SimCity.

I think it would be good if the title made that clear, and didn't pretend this
is some serious simulation. I mean, it's very cool and detailed for a game,
but as you can tell from the video, it's far from perfect. It doesn't care
about collisions, just about a rough approximation of traffic flow.

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corwinstephen
Yup. I'd also like to see a couple other figures alongside traffic flow like,
"total square footage of structure", "collision rate", and "number of
displaced residents."

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rdiddly
Cost might be a good one too.

~~~
bobthepanda
Cost can be very misleading, since most of the cost of big projects like these
tends to be land acquisition.

For example, roads are on average much cheaper than rail lines per lane mile
of construction, but lots of roads get built in rural or depopulated areas
where land is cheaper, whereas rail tends to be built in denser corridors.

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tantalor
Here's a great game that is just junction design:

[https://captaingames.itch.io/freeways](https://captaingames.itch.io/freeways)

 _You are a traffic engineer._

 _Draw freeway interchanges._

 _Optimize for efficency and avoid traffic jams._

~~~
mcphage
I loved this game. As you complete boards, you get to see them all link up and
can watch cars drive all over your city.

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zwieback
In the simpler variants it seemed like some cars passed right through the
middle of semis or were my eyes deceived?

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yeahboats
I don't think your eyes are deceiving you, I saw the same thing. My guess is
(and excuse my incorrect terminology) that the hitbox and the mesh aren't the
same size, so the cars are passing after the hitbox (maybe the cab) safely,
and thus through the trailer mesh/texture.

~~~
RobLach
There is no collision detection here, and consequently no hitboxes. It’s just
a visualization of some simple game simulation data

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yeahboats
There has to be some though right? Otherwise there wouldn't be any back-ups. I
guess they could be point masses that can't occupy the same space of another
point mass + some radius around it.

~~~
RobLach
You just calculate the total load of the road and render enough cars
appropriately with defined positions. Doing that logic through a physics
system would collapse the entire simulation.

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seba_dos1
In case anyone is confused (like me), the video is an simulation made using
mods for the game Cities: Skylines -
[http://www.citiesskylines.com/](http://www.citiesskylines.com/)

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no_identd
Here, two very much related papers, albeit the title of the first one wouldn't
tell you that:

[https://arxiv.org/abs/1612.07888](https://arxiv.org/abs/1612.07888)
Kurauskas, Valentas - On the genus of the complete tripartite graph Kn,n,1
[2017]

[https://arxiv.org/abs/1801.03860](https://arxiv.org/abs/1801.03860)
Kurauskas, Valentas; Šiurienė, Ugnė - Symmetric road interchanges [2018]

And slides from 2010:

[http://web.archive.org/web/20101127223543/http://www.saga-
ne...](http://web.archive.org/web/20101127223543/http://www.saga-
network.eu/phocadownload/auron/Krasauskas.pdf)

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oliv__
That "turbo roundabout" really got me. I feel like for that example (and
probably others) you'd also need to measure the quantity of efficiency lost to
people just trying to understand how the system works and maybe then the
actual score would be lower.

In the same vein, for the few complex systems at the end, they should probably
factor in human mistakes in the score because it looks like it would be
incredibly time consuming to get back on the right way if you took the wrong
turn (and could potentially add traffic to the equation).

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Jiig
My town just recently put in the Diverging Diamond Intersection (or at least
something very similar) on a major highway and major road interchange and the
extra throughput was very noticeable.

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tibbon
I'd like to see it model Powderhouse Square in Somerville, MA... traffic
circle with red lights going both in and out...

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iambateman
I wonder what the theoretical maximum throughput is? Anybody know? The video
got up to 1000, which seems pretty high.

It seems like, from that simulation, that the longer the curve, the higher the
potential throughput. I particularly liked the Turbine.

~~~
divergentSystem
The theoretical maximum would be all vehicles operating at highway speed
limits of perhaps 70MPH, non-stop through all points, bumper to bumper, packed
as densely as possible, according to a profile for vehicular mixture,
achieving was is effectively the equivalent of laminar flow for fluid
dynamics.

If everything is tractor trailers, the throughput is lower based on vehicle
classification. If everything is passenger cars, variation between SUV and
sub-compact car provides a mid-range. If everything is motocycles only the
vehicle dynamic squashes vehicle volume and boosts maneuverability to inflate
apparent throughput.

Somewhere in this, driver skill level and physical fitness comes into play
though, so there has to be an index for that also, based on available
residential zones, and an origin/destination matrix.

If everyone is driving from the retirement home to church bingo, the
throughput will be lower than tractor trailers. Tractor trailers and other
commercial vehicles are expected to operate at lower speeds due to inertial
lag during braking and acceleration, prolonging periods spend at lower speeds.
All other vehicles can only be modeled according to legal speed limits, even
though the accuracy of the simulation is threatened by the fact that in
reality many vehicles will be ignoring speed limits.

Time of day, however, is usually going to model rush hour, which
consequentially alters vehicle profile and driver persona.

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dsfyu404ed
That was an interesting way to kill a few minutes.

My conclusion is that throughput is basically a function of space the
intersection takes up. Also water is wet.

Sure given a fixed space you can optimize type to maximize throughput. If you
really care about maximizing throughput with minimal complexity (the more
rapid fire merges and lane changes you have the greater the chance of an
accident that bottles up the whole system) big rotaries seem to be the way to
go (above/below grade structures and traffic signals are a lot more expensive
and cost more to maintain than some raised medians to divide right turn
lanes).

~~~
nostrademons
In general, but there were some exceptions. The various roundabout solutions
(rotaries, dumbbells, cloverleafs, etc.) seemed to punch under their size,
while the continuous-flow intersection moved a lot of cars in a space not much
larger than a traditional 4-way intersection.

Also, the video showed how effective getting the signaling right can be:
intersections with dedicated left turn lanes (with the TPME mod) outperformed
stock traffic lights by almost 50%. The 4-lane road with dedicated left turn
lane outperformed a 6-lane road without protected lefts.

~~~
mjevans
My own observations were that roundabouts tended to perform better than
intersections with time division utilization and also didn't require any
signaling infrastructure.

Modifications to roundabouts that added dedicated bypass routes for higher
traffic / shorter path (merge to closest output without being part of the main
queue) were inexpensive upgrades.

The same general observation also applied to higher scale variations where the
core loop was either factored out entirely (every route has a dedicated path);
applying more complexity (expensive) lead to better results.

Of course the real world has different needs than this over-simplified
simulation.

Terrain and political situations impose limits on usable work area while use
patterns can vary widely. Worse solutions that used to work in the past can be
woefully incorrect as the world around them changes.

The example that comes to my mind is the pattern in urban areas, especially
like Seattle with heavy geographic constraints that make alternate routes
untenable, where the old "most commuters go in to the city in the morning and
leave in the evening" break entirely as jobs flee the expensive urban core and
leave workers trying to traverse the choke points where cross traffic was
never designed in.

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hjek
The cars move through each other. Looks buggy. But cool.

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mempko
Something about this video was very satisfying.

