
Traffic Simulation - tomcdonnell
http://www.traffic-simulation.de
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reitanqild
Interesting[0] story from a friend of mine who is a _real_ engineer:

Back where he used to live at some point road authorities increased speed
limits and found that average speed decreased. His explanation was simple:
once speed limits matched drivers expectations they started to respect them.
(Other explanations that I can pull from thin air now is police promising that
the new limits will be enforced a lot less leniently than the old, or that
speeds went down because of saturation congestion.)

[0]: although unconfirmed for me IIRC, maybe someone here can tell me were to
find data

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takk309
Traffic engineer here. Generally speed limits are set by measuring vehicle
speeds for about a week. The 85th percentile speed is set as the speed limit.
The reasoning behind this is the assumption that most people will travel at a
speed that is reasonable for the facility. Of course, politics often becomes a
factor and the speed is changed to fit those pressures. Other aspects that can
be taken into account when setting speed limits is the character of the
facility. For example, number of approaches (driveways, other roads, etc.),
type of area (urban, rural), and classification of the facility (arterial,
collector, local street).

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sandworm101
That's the textbook answer. The reality is that most jurisdictions have some
fixed limits depending on the class of road. Parks and such, 30kph. Normal
streets 50. Highways 80, with 90/100 where there are no
ramps/intersections/driveways/people. If we believed in the 85th percentile
then we would have lots of different apparently random limits.

Instead we have limits based on polices very much separate from road
conditions or actual traffic patterns. School zones are 30kph not because
that's the 85th percentile but because that's the law. US highways were once
capped at 55mph to save on gas during the fuel crisis (see 1974 Emergency
Highway Energy Conservation Act). That certainly had nothing to do with
traffic patterns. If 85% was a thing, I wouldn't cross a bridge every day
signed for 50kph but where not a single vehicle is below 80 and the cops say
nothing until you're over 100.

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takk309
You are correct, my response is the text book answer. Often the 85th
percentile is rounded to the nearest speed that matches the fixed limits.
Special speed zones, schools, definitely need to exist for obvious safety
reasons.

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reitanqild
Thanks for chiming in. If you still look at this thread: I guess you live in
the US? (Around here it does not seem to work this way.)

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takk309
Yes, I do live in the US. Even here there is a lot of variation on how speed
limits are set.

A good example happened near my home recently. A road that connects a small
town (population 8,000) to a larger town (population 40,000) was the subject
of quite a lot of debate. The road is a rural two lane roadway that sees about
10,000 vehicles per day. Speed studies were performed by three different
groups (the small city, the large city, and the State Department of
Transportation). Each of the groups determined their own speed limit
recommendation, each was different. Ultimately, the State had final say on the
matter and split the difference between the three recommendations. The State
had originally wanted the highest speed limit of the three. The two towns
wanted lower speed limits citing safety concerns and a desire to force
alternate routes that would take traffic through areas with better
infrastructure for the traffic load.

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Monkey0x9
Very nice, I played a bit around with it but there is 1 thing that bothers me.
The sliders all start at the left but the value it represents isn't at the
left. Some sliders should start in the middle to be right.

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sandworm101
I've seen many of these ring-road simulations over the years. They are great
for pitching lower speed limits, but I fault their assumptions. Traffic does
not feed on itself in this manner. Aberrant drivers and situations are
constantly being inserted and removed from the system. A slowdown at point A
does flow up/down to point B like a wave, but it doesn't wrap around back to
point A to be amplified again.

I do credit this particular simulation for modeling acceleration/deceleration.
Playing around with it leads to the conclusion that many congestion problems
can be addressed by increasing the performance numbers for cars, and by
encouraging people to use their brakes/engines to their full potential. We
should all drive as if in nascar, minimizing gaps to fully utilize the
available road. Clarkson would be proud.

I would like to see a simulation of cars+trucks+motorcyles. Two-wheeled
vehicles are common everywhere and in some countries are dominant on the
roads, yet they are never included in these models. And toss in a few bicycles
just for kicks.

Realworld "simulation": [https://www.youtube.com/watch?v=7wm-
pZp_mi0](https://www.youtube.com/watch?v=7wm-pZp_mi0)

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seanp2k2
And yet, many believe that the best way to reduce traffic jams is to
accelerate as slowly as possible, brake as softly as possible, and leave as
large of a gap in front of them as other drivers will allow.

Normal drivers, unaware of the above, will generally also stop and go slowly
in traffic jams.

My personal pet peeve is when there is a reason for a slowdown (emergency
vehicle, crash, etc) and drivers slow down further to look at it, unaware that
they're are then currently causing the jam they just waited in.

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glaberficken
I love that it has really good model explanations. On my simulations though,
trucks tended to drive extensively on the left lane. In real life trucks don't
usually do this.

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takk309
This brings back some memories of traffic flow fundamentals.

I do remember an assignment that was modeling multiple different traffic flow
models. The thing that stood out to me is how after a long time span with an
network that was stuck with the same vehicles, like this model, traffic flow
issues tended to become exaggerated.

As a traffic engineer, I use a product called Vissim to do exactly this kind
of thing for real world scenarios. Very fun stuff.

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Doctor_Fegg
Brilliant.

Now add an option to import street layouts from OpenStreetMap...

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ck2
It needs a slider for number of 80 year old grandpas who should have been off
the road years ago but DMV keeps passing them to be nice.

Our town has implemented a few of these traffic circles. For some reason a
memorial to the killed appears a few months each time after one is opened. I
don't think it was drivers killed, I think it was pedestrians trying to cross
at the inlets/outlets.

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teraflop
Interestingly, this confirms the common-sense wisdom that coasting, rather
than braking, helps to even out traffic jams.

If you decrease the "comfortable deceleration" parameter all the way, you
significantly increase the number of cars that can be handled without traffic
jams.

Also, try turning T/s0/b all the way down, a all the way up, and then increase
the density to about 75. Because the simulated drivers have very fast reaction
times, the traffic oscillations propagate much faster than the cars are
actually moving, creating some cool-looking spiral patterns.

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andrewfromx
Really well done. I'm not sure how I will use this but I copied the link.

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MarcScott
And irl - [https://www.youtube.com/watch?v=7wm-
pZp_mi0](https://www.youtube.com/watch?v=7wm-pZp_mi0)

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jedanbik
Would taking trucks off of the road actually help this much?

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mxuribe
Not sure that i have a use for this, but its so very cool!

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highace
Welcome to the M25.

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assaflavie
This is so addictive...

