4 ms·

On the subject of counter-intuitive effects in traffic engineering, it also seems that adding more lanes to a highway actually makes traffic flow worse. Wheneve

by wycy 7y ago

On the subject of counter-intuitive effects in traffic engineering, it also seems that adding more lanes to a highway actually makes traffic flow worse. Whenever a car changes lanes on a highway, a small traffic "shockwave" propagates backwards some distance, and more lanes -> more cars crossing more lanes -> more traffic. Anecdata: I live in DC with its notoriously bad traffic. The highways 395 and 495 have 4-6 lanes each and tons of traffic. On the other hand, the George Washington Parkway has just 2 lanes and carries a large volume of vehicles between 395 and 495, but has very little in the way of its own traffic (except in the event of an accident, of course). Traffic only really occurs on the GW at the junctions with 395 and 495 since the X95s don't have the capacity to receive all of the GW cars at-speed. Whereas the X95s will get random backups for seemingly no reason, and at all hours of the day, backups on the GW generally only occur at the ends.

Retra 7y ago
If a lane is shut down due to an accident, it is better to have five spares though.
rpiguy 7y ago
For highly congested areas the increase in the number of lanes is done to accommodate the number of cars on a given route (optimize for capacity). If you can physically fit four times as many cars on a route, but it results in only being able to move at 1/3 normal speed you are still moving more cars over time than a two lane road.
linuxftw 7y ago
It's not random, it's poorly engineered entrance and exit ramps. In order to maintain traffic flow, cars need to be able to increase to traffic speeds on on-ramps and decrease on off-ramps. Much have the beltway system put entrance and exit ramps where they definitely don't belong. Thoroughfares for in-city traffic also don't exist, the interstate has taken that role. To compound the problem, often times entrance and exit ramps are the same piece of pavement.
JamesBarney 7y ago
I think a lot of the counter-intuitive effects in traffic engineering are due to traffic arbitrage. If you make a path much better more people will take it until it's as bad as all the alternate routes.
- rocqua 7y ago
  It can actually be worse. This is called Braess's paradox [1]. The relevant part is: "an extension of the road network may cause a redistribution of the traffic that results in longer individual running times." Further down in the Wikipedia article is an actual example. [1] https://en.wikipedia.org/wiki/Braess%27s_paradox https://en.wikipedia.org/wiki/Braess%27s_paradox
naikrovek 7y ago
Playing "Cities: Skylines" on PC and growing your small village into a metropolis will teach you a lot of things about traffic. More options for cars are usually not good for traffic flow. The drivers in that game don't crash, and otherwise their behavior is greatly simplified, but each vehicle in that game has a start and a destination, and you can follow delivery trucks as they deliver packages and then go back to the Depot to pick up more. It's very fascinating, and may offer us some small window into the kinds of things that traffic engineers deal with without trying to find or write traffic simulation software. And, of course, there are mods to give the player (the Mayor) greater control over lane usage and where stop signs and traffic lights are placed and how they operate. Doing things wrong will quickly result in localized gridlock.
- ptmcc 7y ago
  The biggest flaw in C:S traffic flow, even with the popular traffic improvement mods, is that routes and lane choices are pre-calculated when the vehicle departs and don't really take into account alternative routes/lanes or current traffic load very well or at all. You quickly run into issues where you'll have a 3 lane road that has 99% of the cars backed up in one lane while the other 2 are wide open. Coincidentally it kind of mimics an induced demand problem, but for the wrong reasons. Of course having every vehicle continually recalculate and optimize its path is computationally prohibitive. Given the limitations it does a pretty good job, though!
  - naikrovek 7y ago
    The vehicles aren't situationally aware, you're right. Nothing beyond basic "go when the car in front of you goes, and stop when they stop" kind of thing. That said, there is a non-zero percentage of human drivers that do the same pre-calculated route and lane selection.
  - dawnerd 7y ago
    I found that frustrating too - all the cars piling up into one lane. I'm sure there's some stuff they could have done to pre-calculate and have some "randomness" in there to use the other lanes. There was a plugin, might still be, that kind of fixed it. Maybe I should fire it back up and see whats all new.
snarf21 7y ago
You are absolutely right about the shockwaves. It is also weird that going fast and tailgating actually create slowdowns and make traffic go slower. By not leaving room to slow down, any minor lane change can cause a huge ripple of hard braking and now there is a backlog until everything accordions forward again. I wish newer cars had a constant beeping when you are 10 feet from the person in front of them at 65 mph. We'd actually see better traffic flow. As for parkways, the better flow is because they don't allow trucks. Variance in the speed of cars is what causes traffic. Obviously this is true at on/off ramps. There is no way X95 can take two lanes of high-speed flow of cars into their already packed flow. For parkways with a lot more hills, you can still get slowdowns but these happen almost exclusively just before big hills and at exits to bigger highways (and occasionally at busy on ramps with short merge areas).
- joshuak 7y ago
  People always talk about driving with gaps[1], but I've yet to see a simulator show how detrimental/beneficial these strategies are to individual drivers. It feels like a hawks and doves style ESS (evolutionary stable strategy) problem. It appears to me that the strategy that eliminates the overall negative affect for the group (avoid creating phantom stop signs, by gapping or "always in the middle" strategies) is necessarily unstable. The benefit to any single driver who does not follow that rule becomes increasingly high as more and more others do follow the rule. Even to the point of ignoring a beeping car I should think. I certainly feel the negative effect driving behind someone who creates a gap as more and more cars merge in front of them. In the pathological case the driver who insists on a safe interval between cars will make no progress at all. 1: (CGP Grey) https://youtu.be/iHzzSao6ypE https://youtu.be/iHzzSao6ypE
  - Noumenon72 7y ago
    You say you've observed it, but it doesn't make any sense. If you go 5mph slower than the speed of traffic, you will continuously open up a gap. You never need to slow down farther than that. The cars merging into the gap wouldn't be merging if your lane wasn't faster than their lane, so there's no reason you would have to slow down even farther. What you perceive as repeated slowing down will only happen if you sped back up to match the speed of the car in front of you after opening the gap.

by wycy 7y ago

Retra 7y ago

If a lane is shut down due to an accident, it is better to have five spares though.

rpiguy 7y ago

For highly congested areas the increase in the number of lanes is done to accommodate the number of cars on a given route (optimize for capacity). If you can physically fit four times as many cars on a route, but it results in only being able to move at 1/3 normal speed you are still moving more cars over time than a two lane road.

linuxftw 7y ago

It's not random, it's poorly engineered entrance and exit ramps. In order to maintain traffic flow, cars need to be able to increase to traffic speeds on on-ramps and decrease on off-ramps. Much have the beltway system put entrance and exit ramps where they definitely don't belong. Thoroughfares for in-city traffic also don't exist, the interstate has taken that role. To compound the problem, often times entrance and exit ramps are the same piece of pavement.

JamesBarney 7y ago

I think a lot of the counter-intuitive effects in traffic engineering are due to traffic arbitrage. If you make a path much better more people will take it until it's as bad as all the alternate routes.

rocqua 7y ago
It can actually be worse. This is called Braess's paradox [1]. The relevant part is: "an extension of the road network may cause a redistribution of the traffic that results in longer individual running times." Further down in the Wikipedia article is an actual example. [1] https://en.wikipedia.org/wiki/Braess%27s_paradox https://en.wikipedia.org/wiki/Braess%27s_paradox

naikrovek 7y ago

Playing "Cities: Skylines" on PC and growing your small village into a metropolis will teach you a lot of things about traffic. More options for cars are usually not good for traffic flow. The drivers in that game don't crash, and otherwise their behavior is greatly simplified, but each vehicle in that game has a start and a destination, and you can follow delivery trucks as they deliver packages and then go back to the Depot to pick up more. It's very fascinating, and may offer us some small window into the kinds of things that traffic engineers deal with without trying to find or write traffic simulation software. And, of course, there are mods to give the player (the Mayor) greater control over lane usage and where stop signs and traffic lights are placed and how they operate. Doing things wrong will quickly result in localized gridlock.

ptmcc 7y ago
The biggest flaw in C:S traffic flow, even with the popular traffic improvement mods, is that routes and lane choices are pre-calculated when the vehicle departs and don't really take into account alternative routes/lanes or current traffic load very well or at all. You quickly run into issues where you'll have a 3 lane road that has 99% of the cars backed up in one lane while the other 2 are wide open. Coincidentally it kind of mimics an induced demand problem, but for the wrong reasons. Of course having every vehicle continually recalculate and optimize its path is computationally prohibitive. Given the limitations it does a pretty good job, though!
- naikrovek 7y ago
  The vehicles aren't situationally aware, you're right. Nothing beyond basic "go when the car in front of you goes, and stop when they stop" kind of thing. That said, there is a non-zero percentage of human drivers that do the same pre-calculated route and lane selection.
- dawnerd 7y ago
  I found that frustrating too - all the cars piling up into one lane. I'm sure there's some stuff they could have done to pre-calculate and have some "randomness" in there to use the other lanes. There was a plugin, might still be, that kind of fixed it. Maybe I should fire it back up and see whats all new.

snarf21 7y ago

You are absolutely right about the shockwaves. It is also weird that going fast and tailgating actually create slowdowns and make traffic go slower. By not leaving room to slow down, any minor lane change can cause a huge ripple of hard braking and now there is a backlog until everything accordions forward again. I wish newer cars had a constant beeping when you are 10 feet from the person in front of them at 65 mph. We'd actually see better traffic flow. As for parkways, the better flow is because they don't allow trucks. Variance in the speed of cars is what causes traffic. Obviously this is true at on/off ramps. There is no way X95 can take two lanes of high-speed flow of cars into their already packed flow. For parkways with a lot more hills, you can still get slowdowns but these happen almost exclusively just before big hills and at exits to bigger highways (and occasionally at busy on ramps with short merge areas).

joshuak 7y ago
People always talk about driving with gaps[1], but I've yet to see a simulator show how detrimental/beneficial these strategies are to individual drivers. It feels like a hawks and doves style ESS (evolutionary stable strategy) problem. It appears to me that the strategy that eliminates the overall negative affect for the group (avoid creating phantom stop signs, by gapping or "always in the middle" strategies) is necessarily unstable. The benefit to any single driver who does not follow that rule becomes increasingly high as more and more others do follow the rule. Even to the point of ignoring a beeping car I should think. I certainly feel the negative effect driving behind someone who creates a gap as more and more cars merge in front of them. In the pathological case the driver who insists on a safe interval between cars will make no progress at all. 1: (CGP Grey) https://youtu.be/iHzzSao6ypE https://youtu.be/iHzzSao6ypE
- Noumenon72 7y ago
  You say you've observed it, but it doesn't make any sense. If you go 5mph slower than the speed of traffic, you will continuously open up a gap. You never need to slow down farther than that. The cars merging into the gap wouldn't be merging if your lane wasn't faster than their lane, so there's no reason you would have to slow down even farther. What you perceive as repeated slowing down will only happen if you sped back up to match the speed of the car in front of you after opening the gap.