24 ms·

How traffic actually works

• hammock 13y ago
  That was the clearest explanation of traffic I've come across. I'm inclined to buy into it (often it seems no one knows what they're talking about when it comes to traffic). It also runs counter to what is commonly said about traffic jams. Most people focus on restoring smooth flow by doing things such as driving really slow to create space in front of you. This guy makes the case that "flow" is not really the important factor here. For the record, I am "that guy" who runs all the way down to the end of the closed lane before merging over. Except I don't consider it an asshole move because as OP points out, I'm making the best use of the road ahead (while allowing fewer people to merge ahead of me)
  • Dirlewanger 13y ago
    Except I don't consider it an asshole move because as OP points out, I'm making the best use of the road ahead (while allowing fewer people to merge ahead of me) Which solves nothing but to piss off everyone else behind you who has waited because ultimately someone will let you merge in (which they flat out shouldn't) and said behavior will only encourage others to do so in the future which inevitably will piss off even more people.
    • protonfish 13y ago
      Nobody is forcing you to stay in the congested lane. It was your choice and you have no reason to think everyone else has to do the same. If it's a legal lane, then drive in it.
      • Fuzzwah 13y ago
        Its a legal lane to no where. By using it you're then relying on the good graces of one of those chumps you just passed. They don't need to let you back in. They're in the legal lane to somewhere, you just got no where really quickly.
        • protonfish 13y ago
          You don't have to let me in. I am sure someone in front of you will. Then you have the right to throw a little hissy-fit because of having to live with the repercussions of your poor life decisions.
          • Fuzzwah 13y ago
            My response isn't to throw a hissy-fit. I catch public transport to work. Mostly because I'd rather have a nap on the way to and from work than deal with wankers like you.
      • zenzen 13y ago
        That's fine if you can safely merge into the exit lane without slowing down traffic in the non-exit lane. Here's when it's not OK: cars in the exit lane are traveling at 2mph. You drive up next to the line, slow down from 55mph to 2mph and wait with your blinker on for someone to let you in. Now you've slowed a second 55mph lane down to 2mph, blocking thru traffic and creating a dangerous stoppage in the non-exit lane. Seriously, don't do this.
        • notacoward 13y ago
          I see this all the time in Massachusetts - Route 2 West to Route 95 North, about a mile from my home. At rush hour there are always enough jerks playing the "merge at the last minute" game that only the left lane is still moving significantly, and sometimes not even then. Every once in a while there's an accident because people who were unaware of this "congest all the lanes" behavior didn't see a blocked left lane soon enough. You're right, it's a major dick move. It's reckless and endangering; police should treat it accordingly.
          • j-kidd 13y ago
            Perhaps a more advanced camera technology that detects asshole driving behavior instead of speeding? Rich dudes pay to get ahead. Increased revenue for the police department. Improved traffic. Win-win for everyone.
        • mavhc 13y ago
          Have you blocked through traffic? Only if the number of cars per second drops. Also as there's going to be a queue of, say 1000 cars behind the merge point, it's better than the queue doesn't extend backwards into some other area of traffic, causing people who are turning off before the merge point to become involved in the slow traffic
    • StavrosK 13y ago
      Yep. It's just like saying "I just jump to the front of the queue because I'm minimizing time spent waiting". Yeah, for you, whereas everyone else has to wait N more seconds.
  • protonfish 13y ago
    Smoother traffic flow (as opposed to stop-and-go) may get you to your destination slightly slower due to added distance between cars, but that is not the only consideration. Gas mileage and car wear-and-tear are improved (and I suspect safety as well) when the driver is not alternately stomping on the accelerator and brakes.
  • tucaz 13y ago
    This is the one thing that really pisses me off. It gets worse when the lane that you are merging into has a line of cars going back almost a kilometer. Doing this is like saying "Hey you idiots, look how smart I am" to everyone waiting for their way in the right lane.
    • cecilpl 13y ago
      What pisses me off is having to wait in a kilometer-long line of people merging when what I really want to do is turn right down a side street. Please keep that line as short as possible by using all the lanes available.
      • jessaustin 13y ago
        I thought we were talking about freeway/tollway/highway traffic? If there are side streets, that's a different situation.
      • bryanlarsen 13y ago
        What pisses me off is having to wait in a kilometer-long line of people blocking all available lanes when what I really want to do is turn right down a side street. Please leave exit lanes open by merging as soon as possible.
    • HelloMcFly 13y ago
      I don't know why people get so mad about this. You use the term "right lane", but what does that mean? It only means what you want it to mean in a given situation. The lane is open until it is not, and as long as both lanes are open they are both "right" (in the non-directional sense, obviously). Why rely on clearly nebulous (sometimes variable, sometimes non-existent) social norms rather than concrete traffic norms? Use the open lane, or don't complain when others do. It's not asshole behavior, it's common sense driving. There is no reason to take offense while driving so long as people 1) follow the rules of the road and 2) do not endanger others with reckless driving. Using open lanes does neither, and it only inconveniences the people that refuse to use the roads as they are intended to be used.
  • libria 13y ago
    Part of the problem is a lack of established convention. When a theme park opens its gates in the morning, we expect a large crowd to jostle in through a constricted entrance like a funnel. However, when we queue up at the bank, if the line extends past the cordon, nobody runs up to that point and stands shoulder to shoulder with other patrons; we maintain a single file. Which of these rules applies to cars merging? There isn't a set standard, so people invent their own and perceive their rule is universally understood.
    • dvanduzer 13y ago
      There is a protocol. Even in heavy traffic, if you observe the following distance that the DMV tells everyone to observe when taking a driving test there is always room to merge.
      • jessaustin 13y ago
        ...following distance that the DMV tells everyone to observe... Who on earth leaves such a distance in stop-and-go traffic? I certainly haven't seen it. You might as well just assume that every driver will be safe, considerate, and wise.
        • danielweber 13y ago
          If traffic is stop-and-go, a zipper merge is trivial.
          • jessaustin 13y ago
            Ummm, not in my experience.
  • jessaustin 13y ago
    For the record, I am "that guy" who runs all the way down to the end of the closed lane before merging over. Except I don't consider it an asshole move because as OP points out, I'm making the best use of the road ahead (while allowing fewer people to merge ahead of me) In interstate travel I have noticed the increasing frequency of the "two trucks" maneuver, in which a big rig driven by someone philosophically opposed to your habit will drive in the "closed ahead" lane directly alongside another big rig in the "open but crawling" lane. Once the two trucks near the point of obstruction, the "open" driver is happy to let the "closed" one merge in, and there is no merging at the point of obstruction, which is both a safety and traffic-flow win. I contend that the two truckers have done more to speed actual traffic flow (i.e. from a point before the obstacle to a point after it) than any number of assholes have. While at the beginning of the maneuver there is a solid line of fuming assholes behind the "closed" truck, by the time the obstacle has been reached all fuming asshole traffic has completely merged behind the "open" truck. Since it is only merging at the point of obstruction that slows traffic more than the obstruction itself already has, this is a net win. Of course this maneuver must be repeated by successive pairs of truckers, and occasionally an asshole will "win" by racing forward between pairs, but that's minor.
    • danielweber 13y ago
      I wonder if good zipper merges would be more likely to happen if, instead of having lane 1 merge into lane 2, both lanes got merged into lane 1.5.
      • jessaustin 13y ago
        Yes, that probably would improve things. I could totally see that working if we had electronic (or other quickly reconfigurable) lane markings.
  • kmm 13y ago
    > For the record, I am "that guy" who runs all the way down to the end of the closed lane before merging over. Except I don't consider it an asshole move because as OP points out, I'm making the best use of the road ahead (while allowing fewer people to merge ahead of me) Asshole move? I don't understand. I don't live in the US, but here it is encouraged to act like this! It's called a zipper merge. I remember public messages on the TV encouraging this behaviour, and there's road signs near merging lanes. What's so asshole about this?
• themodelplumber 13y ago
  Great. I really hope that like 100% of jerk drivers do not find out about this. Never let people merge in front of you...geez man, really?
  • jliszka 13y ago
    That was a joke ;) Onramp regulators are the real answer, letting only 1 car merge into traffic at a time at a rate well under 1 every 2 seconds.
    • josefresco 13y ago
      Doesn't this simply displace some of the wait to the onramp?
      • steamer25 13y ago
        I used to be skeptical of ramp metering until the day I drove several miles below the speed limit to 'celebrate' the anniversary of a ticket I'd received. Instead of driving 14 MPH over the limit, I drove 14 MPH under the limit (legally that time) as a form of radical civil obedience* . As expected, this caused quite a bit of unnecessary congestion. It actually turned out to be harder to sustain a backup* * once I entered the metered areas. The reason? Metering limits the number of idiots on the road at a given time. I.e., my attempt at being intentionally stupid was mitigated because the meters prevented the road from nearing saturation. The little bit of wiggle room on the road made it relatively straight-forward to just route around me. So yes, in one sense meters merely move the wait from one point to another but they also remove some of the interactive effects of human stupidity. * https://www.youtube.com/watch?v=OoETMCosULQ https://www.youtube.com/watch?v=OoETMCosULQ "I know no method to secure the repeal of bad or obnoxious laws so effectual as their strict construction." -- President Ulysses S. Grant's First Inaugural Address * * No I did not intentionally prevent people from passing me. My beef is with the revenue centers that are the pre-emptive enforcement of low velocities--not with my fellow passer-motorists.
    • isaacaggrey 13y ago
      > That was a joke ;) I found your article interesting in light of the popular traffic piece you mentioned, but ending it with jokey suggestions after a fairly serious analysis doesn't really make it clear if you're joking or not. But hey, maybe @themodelplumber and I are the only two who thought you were serious.
• bryanlarsen 13y ago
  The central assumption for this argument is that the average time between cars is fairly consistent no matter the environment; small changes to this time do not matter. However, on a congested road very small changes can have huge effects. IIRC, one study showed that a 1% decrease in the number of vehicles resulted in a 28% decrease in commute time.
  • daniel_levine 13y ago
    This. I've studied traffic extensively and if you ever wonder how it works, it is actually quite similar to any other network. Imagine road traffic is the same as traffic through your servers. There are hard bottlenecks at various points, but in practice what matters is planning for variability. Real-life situations like someone cutting you off, people rubber-necking, the natural commute tendencies etc tend to be much more impactful. Unlike with servers, you can't just spin up a new lane, and "packet loss" is pretty much unacceptable :) EDIT: For all you infra folks out there, imagine not being able to dynamically provision resources, no packet loss and worse, the packets can basically do what they want. That includes things like crashing into each other and stopping any other packets from making it through.
    • jameshart 13y ago
      re spinning up a new lane: the A38-M Aston Expressway into Birmingham, UK dynamically multiplexes its lanes together into variable width channels depending on the required upstream/downstream bandwidth :) - http://i3.birminghammail.co.uk/incoming/article1294911.ece/ALTERNATES/s615/astonexpressway-1294911.jpg http://i3.birminghammail.co.uk/incoming/article1294911.ece/A...
      • aroch 13y ago
        That is sort of terrifying but also quite cool
      • CodeCube 13y ago
        aww hell no. I'd be terrified of merging into that lane with no highway divider between me and oncoming traffic. Small two lane highways are bad enough! :P
• jameshart 13y ago
  The catastrophe theory model seems compatible with the anti-traffic solution to traffic 'standing waves' - by slowing down a little when approaching stationary traffic, allowing that stationary blockage time to 'evaporate', you avoid slowing down below the catastrophic threshold that puts your rear bumper more than 2 seconds behind your front bumper, keeping the road in the higher capacity regime. Same reasoning is why UK motorways signpost 'advisory speed limits' - and some even have variable enforced speed limits - to manage road capacity.
  • jliszka 13y ago
    OP here, excellent point. Since so much is determined by occupancy (cars per km), I think the only way to switch domains is to lower the road's occupancy, which is exactly what you're doing when you leave extra space in front of you. However, I tried to point out that this probably comes at the expense of increased occupancy somewhere else, so you're just moving the jam around.
    • jessaustin 13y ago
      ...so you just moving the jam around. If there is a reliable way to move it behind me, I'd like to know!
    • hamburglar 13y ago
      One thing your analysis leaves out is the human/psychological element. I've never bought into the idea of "erasing" the waves causing an overall throughput increase directly, but rather that it adds buffers so the traffic is a lot more predictable. The more people need to slam on their brakes, the more they deviate from the optimal-flow following distance, effectively wasting throughput. I guarantee you that when humans are involved, you'll get a lot better throughput from a line of nicely synched cars humming along at 25mph with a 2 second gap than the same cars constantly speeding up and slowing down, but maintaining an average speed of 25mph and attempting to maintain a 2 second gap despite wildly varying traffic conditions.
    • underwater 13y ago
      But if you can spread the jam over a longer area with higher speeds then you should be able to prevent any part of the jam hitting that catastrophic threshold.
• durandal1 13y ago
  For a blog post that that claims to explain "How traffic actually works" I find it disappointing that the author doesn't even briefly touch on the dynamic properties of this problem. No, this is not how traffic works, the model is much more complex and you're missing very large dynamic effects.
• petenixey 13y ago
  This is such interesting stuff. If you go just one step further and plot the carrying capacity of a road against the speed of traffic on it you get a hump which tends towards zero as speed reaches infinity. The significance of that hump is that it represents a maximum carrying capacity for a road - an optimum speed. If you do the numbers I believe it's about 50mph. What I think is really cool is that when the road is completely at-capacity, traffic is stable above 50mph (as cars can just slow down and capacity increases) but collapses to a jam below below 50mph (because as the traffic slows down to absorb new cars, capacity falls still further). This is way why, when the M4 motorway funnels down to only two lanes as you approach London, there is a 50mph speed limit - it literally allows the road to carry more vehicles per hour. I also suspect that it's another benefit (other than safety) of setting 50mph speed limits in contra-flows and during roadworks - it allows the road to carry more vehicles despite having fewer lanes.
• exue 13y ago
  The purpose of making traffic flow smoother is that it'll push out the point where flow breaks down and you can extend the zone of (relatively) uncongested operation with higher vehicle throughput. Smoother flows leads to fewer abrupt stops and rear-end accidents that make the wait even worse. (Cars in a 'beautiful line at 35mph' behave differently from stop-and-go and for instance a fleet of self-driving cars would make the highest throughout, and extreme example) As for late merging, evidence shows that it's more efficient in congested areas and by reducing the line, you help prevent spillover to other areas and importantly accidents due to conflicts. I can see one super suboptimal situation where a car completely stops to change lanes (into a stopped/slow one) due to bad flow, resulting in more than just a 2 second impact on one lane. MN signs say "use both lanes / take turns" and dynamically adjust based on traffic conditions. In LA, the "zipper merge" seems to be standard for most construction zones in traffic jams, with the only distinction being where you merge in the last 200 feet http://www.dot.state.mn.us/trafficeng/workzone/doc/2004DLMS-Evaluation.pdf http://www.dot.state.mn.us/trafficeng/workzone/doc/2004DLMS-... http://www.dot.state.mn.us/trafficeng/workzone/doc/When-latemerge-zipper.pdf http://www.dot.state.mn.us/trafficeng/workzone/doc/When-late...
• meatwad 13y ago
  Thanks for explaining this for us. What is missing from this compared to what I see on the roads is trucks. The truckers always leave large gaps and it's a sure bet that jerks will cut in front of these rigs. I don't think the trucks are leaving that much space to "smooth out" the traffic but it's more a matter of safety and efficiency (saves fuel) when all the nimble passenger vehicles are stop and go and zipping all over the place. Truck drivers are so patient, they have to be, especially in rush hour I've seen cars jump in front constantly and the trucks keep making more space for them. The other half of people seem to just follow at the same pace instead of cutting in front, and every time some one does cut in front it slows them down another 2 seconds. So my theory is aggressive drivers aren't much affected by trucks but easy drivers are slowed down because they won't get in front... But if nobody gets in front of the trucks then everyone makes it at the same time. Basically the trucks are blockers for the aggressive drivers, so if you use them they'll get you home ahead of others. I don't imagine any truckers are on HN?
  • timthorn 13y ago
    If ever I hit a jam on the motorway, I always pull to the left lane (UK), join the trucks, and imitate their driving style. Almost always, over the length of the jam, I beat my neighbours in the middle lane without doing any lane hopping.
• methodin 13y ago
  I thought the whole point of these "fixed traffic jam" articles was to make your undoubtedly terrible commute go faster by giving you something to do, or at the very least providing a false sense of control over a situation (like a superstition). The science behind traffic is completely arbitrary and nonsensical no matter how many graphs or numbers you put behind it. There are simply too many different drivers out there to make any empirical evidence useful.
  • privong 13y ago
    I think the idea is to give it a treatment analgous to how statistical mechanics works for properties of gas, for example. True, we can't easily say what individual atoms/molecules might do, but we can say something about the gas as an ensemble. It's true that free will may make drivers tougher to predict than individual molecules. But molecules obey quantum mechanics and one could conceivibly construct probability distributions for how that particle might behave. In principle, probability distributions could also be constructed for the behavior of individual cars/drivers, so a statistical approximation for the behavior of the ensemble of drivers in a system is probably a reasonable aim.
    • methodin 13y ago
      I get that we want it to be consistent but having driven the same roads for years, in the same times of day, in varying but similar weather patterns the driving is completely sporadic so no amount of normalization well give you anything more than a crapshoot on any given day other than "it will probably take me anywhere from 20 to 50 minutes." Statistical analysis is great for many things but understanding the "now" of a traffic pattern is not one of them.
      • privong 13y ago
        I think it depends on what one wants to model. For a general understanding of how traffic flows, a statistical model is okay. On the other hand, if you want to know why a specific traffic jam is happening and how it's affecting the flow, a statistical model won't be sufficient.
• johlindenbaum 13y ago
  I'll have to try and find the article, this was years ago, but there was a study about people just tapping their brakes lightly that would cause the person behind to do the same, a little longer, etc. etc. that would cause a big traffic jam over time. The study calculated some figure that showed if people just kept their distance up to the person in front, and instead of lightly tapping the brake as a reaction to a merge, and just let the car idle a bit to slowly decelerate, you could reduce traffic slows / jams by a lot. This seems more reasonable than the solutions presented by this article, don't let people merge.. really?
  • dvanduzer 13y ago
    I believe the article you seek is the one being critiqued by this author. Currently I drive for about 15 miles on a major highway (during rush hour at least once) on most days of the week. I rarely need to use my brakes.
• unoti 13y ago
  There are cases where leaving a greater-than-normal lane spacing makes sense, in the interest of safety. Safety is something utterly ignored in the fascinating analysis in this article. For example, if I'm in a left lane buzzing along at wide-open freeway speeds and the right lane next to me is congested and backed up and not moving, I leave more than normal spacing in front of me. Why? Because it's very likely that someone stuck in the right lane is going to dart out in front of me or the driver in front of me, and very likely that I'm going to need to make a very sudden stop. It's just unsafe to cruise along at 75 mph next to hundreds of cars that are looking for an opportunity to jump in front of me. The prisoner's dilemma also applies to this. Behavior that is "optimal" for a single individual in terms of getting to destination is not what's optimal for the collective in terms of safety. And optimal behavior for getting to destination is not the optimal behavior for safety or lower stress driving, either.
  • exue 13y ago
    I'd argue that safety directly translates into speed as well, as a single accident can shut down 1-all lanes of the freeway and result in several hours of delay for the parties involved. So even being selfish you'd have reason to err on safety's side
  • bostonpete 13y ago
    You probably shouldn't be cruising at 75 mph next to a lane of stopped traffic regardless. Leaving more space between you and the next car wouldn't seem to make that practice significantly safer. I agree that the optimal behavior isn't universal though. One example I'd cite is that I always leave more space when the vehicle in front me is so big that I can't see what's in front of it since that limits my reaction time to what that one vehicle is doing rather than what's going on further ahead.
  • DanBC 13y ago
    Shouldn't you leave less space, to dissuade people from jumping in the gap?
    • dvanduzer 13y ago
• mmoche 13y ago
  Isn't the real problem with traffic more perception than reality? I'd be much more interested in how much additional stress/unpleasantness is caused by stop-and-go traffic that would be alleviated with the concepts mentioned in the "wrong" article.
• joosters 13y ago
  What an arrogant article. How can you title something 'How traffic actually works' when you don't examine actual traffic. Instead, make up a simple model, run some code, claim it shows what you want. It's not like modelling traffic is so insanely hard that we need such simplistic models. Pick as complex a model as you like; we have the computing power to simulate it!
  • leothekim 13y ago
    If you RTFA, he compares the results of the model to actual data collected by the Federal Highway Administration. Why use a complex model when a simple one suffices?
    • msane 13y ago
      Not a fan of the use of "RTFA".
  • analyst74 13y ago
    What author is trying to argue against is the idea of "smoothing-the-wave", which he does prove in his overly complicated article. Yes, there is too much rambling for making a point, yes the tone of title is quite arrogant / link-baity. But this terrible essay did prove the point: smoothing the wave is not going to make you go faster.
    • dvanduzer 13y ago
      He utterly fails to make that point. His simple model explains the idea of maximum throughput / congestion. It utterly fails to explain the extreme variance in real traffic throughput during congestion. The original article does.
      • jliszka 13y ago
        OP here. That's a really good point. I just think the original article overreached in saying that zipper merging is a "simple cure" for traffic jams, without taking into account that the road might at or near capacity (and you hit capacity in a hurry when you lose a lane). My article doesn't account for variance in flow rates in congested traffic, but variance in car length (% of trucks on the road) might explain it. I'm not convinced that merging behavior is the culprit; the original article only speculates that that's the case. I'm putting forward a reasonable explanation for why that isn't the explanation, and a basis for evaluating whether it might be — in particular, whether a zipper merge results in higher flow rates after the bottleneck.
• ynniv 13y ago
  Since occupancy determines flow rate, there’s not much benefit to trying to “cancel out” a traffic wave This is so fundamentally mistaken as to make the rest of the analysis useless. Traffic waves account for the majority of highway traffic (excluding on and off ramps), and cause the flow rate for a road to be decimated. It should be obvious that avoiding rate decimation is (literally) an order of magnitude more effective than preventing people from merging in front of you. Road signs commonly ask you to use both lanes up to the point of the bottleneck. That’s reasonable advice, but it’s not going to get anyone home faster. No, but if your line is twice as long it is twice as likely to cause contention on the roads behind you. ie, if an exit road has a traffic line twice as long, it is probably causing traffic on the non-exiting highway. So, do everyone a favor and (a) don't tailgate (b) ignore people merging in front of you (c) don't drive in rush hour, and (d) ignore traffic advice from people who don't drive. (http://tomvanderbilt.com/traffic/the-book/ http://tomvanderbilt.com/traffic/the-book/ has actual good advice)
  • hnrandom 13y ago
    >Road signs commonly ask you to use both lanes up to the point of the bottleneck. That’s reasonable advice, but it’s not going to get anyone home faster. If you simply kept reading: >This is good because it is less likely to affect other traffic by spilling out onto onramps and surface roads. Also maybe there’s someone on the highway who’s planning to exit 3 miles before the bottleneck. If the backup is 2 miles instead of 4 miles, that person doesn’t have to wait in traffic.
    • ynniv 13y ago
      His point is a little different: that someone exiting the highway might exit before reaching the traffic if the line were shorter. My point is that a line on an exit ramp which reaches back onto the highway will cause additional traffic for everyone on the highway. Since his model doesn't take traffic-causing-traffic into account, he never reaches my conclusion. Exit lines are particularly costly because adjacent lanes of traffic generally only sustain a 10MPH difference in speed. Once the right lane is stopped, a 4 lane highway has a maximum safe speed of 30MPH in the left hand lane. This reduces the flow capacity of the highway and may cause the left lane to lock up because left lane drivers get frustrated at slow speeds and are the more likely to tailgate and cause traffic waves. His model also is only concerned with the self: "can I reach my goal any faster", and he reaches the mostly correct conclusion that he basically cannot (even though avoiding traffic waves by driving in the rightmost non-exit lane would help him significantly). He does not appear to value his contribution to the delay of the people behind him, which is where all the really interesting conclusions are found, including the reason why traffic wave busting is beneficial.
• StavrosK 13y ago
  I don't understand the claim that front bumper to front bumper is a bigger problem the faster you go. When you're going fast, the 4 meters of a car are much shorter (in terms of time), because 4 meters at 4 m/s is a much smaller amount of time than 4 meters at 0.2 m/s.
  • maxerickson 13y ago
    It says it is a bigger problem at low speeds.
    • StavrosK 13y ago
      ...oh. Wow, major reading comprehension fail there. Thanks.
• chris_wot 13y ago
  I'm wondering what if I'm missing something here: "If anyone tries to tell you that if only drivers left space in front of them and took turns merging, traffic would flow smoothly, and it’s only because of jerks that there are any traffic jams at all, just ask them what’s going to happen at the next merge. Where is that extra space going to come from? You cannot keep 2 seconds back from the car that has just merged in front of you without, um, slowing down. If the car in front of you is also slowing down for the same reason, you have to slow down even more. This is basically the definition of a traffic jam." However, if you have two lanes, with one lane blocked, then wouldn't not allowing for another car to safely merge into your lane prevent them from merging? Then you have one lane moving quite fast, but the other lane would be going more slowly... Like I say, there may be something I'm missing here, I'm happy to be enlightened!
• jwallaceparker 13y ago
  I wish more drivers would follow the driving etiquette that advises one to "stay right except to pass." On every highway trip I see countless cars camped out in the left lane but moving at the same rate as the rest of traffic. One should only use the left lane for passing. Otherwise, stay right. Traffic flows smoothly when people follow this system.
  • privong 13y ago
    > I wish more drivers would follow the driving etiquette that advises one to "stay right except to pass." In many places this is the law, rather than just ettequite. :) Of course, that doesn't mean people actually stay right unless they're passing.
  • jff 13y ago
    It's evolved sufficiently here in California that I believe Californian drivers now prefer to pass on the right rather than the left, given a choice.
  • thrownaway2424 13y ago
    In practice this cannot work. "Except to pass" allows for any nonzero differential speed. Someone will be doing 55MPH in the rightmost lane and someone will pass them at 55.1MPH, and someone will pass them in the left lane at 55.2MPH.
    • jwallaceparker 13y ago
      This is fine as long as each driver migrates back to the right as soon as feasible.
  • CWuestefeld 13y ago
    I blame The Eagles. By popularizing the meme of "fast lane" in preference to that of "passing lane", they've badly damaged the laminar flow of highways, and likely contributed significantly to road rage.
    • 13y ago
• szirka 13y ago
  I honestly believe it's the quick decrease in speed by drivers that causes non-accident traffic jams during rush hour. Don't hit the brakes before merging and when someone merges in front of you, you should have kept enough space so you do not need to hit the brakes. Really, I think traffic would move much smoother if drivers leave at minimum 2 car lengths of open space between the vehicles in front of them and only hit the brakes on the highway when it is absolutely necessary. If all the cars are put to a halt because of quick and hard braking cars ahead of them, its going to cause a chain reaction that is way more time consuming to drivers behind you than the author's "2 seconds slower to destination" factor due to a merge in front of you.
• polynomial 13y ago
  Given that ants don't have this problem, won't driverless cars be able to adopt ant-like traffic protocols to eliminate congestion? cf. http://www.phschool.com/science/science_news/articles/ant_traffic.html http://www.phschool.com/science/science_news/articles/ant_tr...
  • samatman 13y ago
    Ants have no momentum, to a first approximation. It's not at all clear their algorithm would scale.
    • lisper 13y ago
      > Ants have no momentum And collisions between ants are not catastrophic.
• acomms 13y ago
  "At the risk of being helpful, here are some things YOU can do that are actually guaranteed to improve commute times for everyone: ... 2. Don’t let people merge in front of you, ever." How will the commute times of those who sit there waiting to merge be improved? If traffic flow was heavy enough they would never merge. If they didn't pack a sandwich they'd starve!
• GigabyteCoin 13y ago
  The article claims that the maximum theoretical throughput for traffic per lane is 2,400 vehicles per hour. What if every vehicle is separated by 2 seconds of stopping time, but all vehicles are moving at the exact same speed of 200 Miles per hour? It would take one vehicle 0.005 hours to cover a mile (5,280 feet). Assuming each car had a length of 10 feet, it would take 0.005/528 = 0.0000095 hours for a vehicle to cover it's own length at 200 mph. 2 seconds is the equivalent of 0.0006 hours. So each vehicle would take approximately 0.0006095 hours to cross any given point. Providing a number of 1/0.0006095 = 1,640 cars per lane per hour according to my calculations. I expected the number of cars per lane per hour to increase with speed, which is why I did the calculations, but apparently it does not! If you could actually drive 200 mph on your way to or from work however, you would get there sooner than you typically do in traffic. It's an odd problem to think about.
  • tonyarkles 13y ago
    I love it! It's a counterintuitive result, but if you think about it, it makes sense.
  • sk5t 13y ago
    Hmm? This shouldn't be surprising at all given the rather large assumption that one car passes a fixed point each two seconds. Go as fast as you like... an observer on the roadside must still wait two seconds for each car to rocket (or crawl) by. Imagine having an ethernet card that could only send one frame a second to a party on the other side of the globe: would you care if the additional network latency due to switches and the speed of light were 1msec or 200msec? Where we may see great improvements in road throughput is in driverless cars that greatly reduce that gap.
    • GigabyteCoin 13y ago
      Yes, that seems to be the only possible metric we can reduce to increase the flow of traffic. Cut that number in half from 2 to 1 seconds, and you would approximately double your throughput.
    • dclowd9901 13y ago
• susi22 13y ago
  This article is so horribly incorrect and fails right at the very beginning with the wrong assumptions and a idea that isn't thought through. What the author forgot is to take by far the most important variable into account: SPEED So let's quote the author: "The important fact: there is a limit to the number of cars that can pass by a given point on the highway in a given amount of time, and that limit is one car every 2 seconds, per lane. So imagine you are in heavy traffic during rush hour. There are a certain number of cars in line in front of you. Let’s pick a point on the road to call the front of the line — say, the point at which you plan to exit the highway. The line gets shorter by one car every 2 seconds. If there are 1,000 cars in front of you, it’s going to take a minimum of 2,000 seconds for you to get to the front of the line. It doesn’t matter whether people are kind and let cars merge in front of them, zipper-style. It doesn’t matter how much stop-and-go there is. The simple fact is that it takes 2 seconds per car for you to get to the front of the line, and there are some cars in front of you that have to get there before you do." Ok this is correct. BUT: I don't care if it takes me 2,000s if my exit is 44 miles away (80mph) but I do if my exit is 10miles away. So you drive along at 80mph and there is cars around you everywhere. Do you care? No, you're covering distance. It's the same as if you were all by yourself on the highway. The distance between cars is absolutely irrelevant and so is the time between cars if you still go 80mph. Nobody would call it a traffic jam. Next quote: "Leaving space in front of your car for people who are trying to merge won’t solve anything." Yes it absolutely does. And it again solves one very very important piece of a traffic jam: Average speed. Leaving space is crucial. It allows people to do two things: - Accelerate - Not having to brake These two are crucial to get speed up and thus resolve traffic jams. Incidentally he linked an article which visualizes exactly this: http://www.smartmotorist.com/traffic-and-safety-guideline/traffic-jams.html http://www.smartmotorist.com/traffic-and-safety-guideline/tr... (Section: Merging-lane Traffic Jams, A Simple Cure) You can see on the right side how his "cars per second" is _exactly the same_ than on the left side. Yet, everybody will be much happier on the right side. Why? Because the speed's been increased and people in the cars cover ground. Next quote: "Suppose you’re on a 2-lane (each way) highway and one lane is closed up ahead due to construction. Now the flow rate of your lane is cut in half (or there are twice as many cars in line in front of you, depending on how you want to look at it). Road signs commonly ask you to use both lanes up to the point of the bottleneck. That’s reasonable advice, but it’s not going to get anyone home faster" Yes it absolutely does. Why? We are all programmers here so let's talk in our lingo: This is our API. Our interface that we agree on. If people follow the interface (rule), then we have people agree on it and we are predictable and prevent braking. This is exactly what we want. Increase speed (with gaps), prevent breaking, be predicable. No surprising cars merging means no breaking and thus increase in average speed which in turn means no more traffic jam. Please, disregard this article. The conclusions are wrong, the assumptions are wrong and the deductions and the terrible advice he gives at the end is wrong.
• pertinhower 13y ago
  As a matter of general policy, if you're going to declaim another article as wrong, you really should bring clearly stronger evidence—generally in the form of citations—to the table. Otherwise it's just "he said she said" as in this instance.
• jere 13y ago
  I'm having a very hard time accepting the claims of original article, "The Physics Behind Traffic Jams." Recently, I've tried to perform a few basic hypermiling techniques, especially "timing a light": if I'm the first car to encounter a red light, I leave extra space and can sometimes be moving through the light at 20mph or more when it turns green instead of stopped. It seems analogous to leaving space in front of you during a traffic jam, so that you can exit the jam at a faster speed. The problem with this is that other drivers will often take advantage of your behavior. They think the most appropriate thing to do is race up to the red light. So instead of profiting from the faster speeds out of the light, they instead cut you off and start the light cycle at a predictable 0mph. You haven't done anything except swap places with these jerks.
  • pertinhower 13y ago
    ...which also handsomely shows the real complexity of modeling systems where the human mind is involved, and the folly of over-simplified "1 car per 2 seconds" systems that look good in a computer model but fail to capture the real experience or decisions of piloting a car.
  • cbr 13y ago
    Vehicle sensing systems mean the basic assumption of light timing no longer holds: when you get to the light can affect how long you have to wait.
    • jere 13y ago
      I understand, but I'm not making the assumption. I'm talking about roads I drive everyday that are clearly on timers and I've even got good estimates on how long the cycles last.
  • losvedir 13y ago
    Flying through a light that just turned green for you is a good way to hit a pedestrian or someone coming in from the left or right trying to catch the tail-end of their yellow. Please don't do this.
• Lagged2Death 13y ago
  Don’t let people merge in front of you, ever. When you merge, you're merging in front of somebody, so a rule like this, universally followed, is effectively a ban on merging. How's that going to work, exactly? Travel time isn't the only consideration. Things like safety, fuel consumption, and stress levels are important too.
• DigitalJack 13y ago
  This article accounts for some of what causes traffic jams, but have a look at this video. It shows a study done where people tried to drive on a circular track at a constant speed. Things start out okay, but it quickly devolves into the back propagating wave. http://www.youtube.com/watch?v=Suugn-p5C1M http://www.youtube.com/watch?v=Suugn-p5C1M
• Evolved 13y ago
  I've tested this theory and others on my commute. Whether I speed up and never let anyone in (nearly impossible since someone won't care and will merge knowing you'll most likely not hit them) or keep a good 7-10 car distance, it doesn't matter. My commute on the freeway with either of these two ways or on side streets is always 45 minutes +/- 1 minute. The difference is I'm using less gas and putting less wear on my clutch, brakes, etc. by leaving a big gap between myself and the cars in front of me. Furthermore I find that my demeanor is much more calm and relaxed when I leave a big gap because I'm not tensing up to avoid rear ending someone every time I stop.
• Evolved 13y ago
  Shockwave effect is a big contributor to traffic. Even moreso than accidents/road construction.
• tlrobinson 13y ago
  I suspect that a critical mass of self-driving cars will improve traffic flow for everyone. I don't have a clue what the percentage to achieve critical mass is, but my wild-ass guess would be 20-50%.
• deleted 13y ago
  [deleted]
• maplebed 13y ago
  My two favorite parts of this article: the author lives in New York and "Don’t let people merge in front of you, ever." Now I understand New York drivers!
• orthecreedence 13y ago
  "Don't let people merge in front of you, ever." Californians have this one covered.
• amalcon 13y ago
  Everyone has their own pet theory of how people should behave in traffic. The problem isn't even that most of them are wrong -- most of them are "correct" in the sense that, if everyone behaved in this way, traffic jams would be less common and have shorter durations. The problem is that you don't know which theories other drivers have, so you don't know how they will react to different situations. If you don't know how other drivers will react, you need to drive conservatively -- which usually means slowing down. Take a construction merge down to a single lane. It might be the case that it's best to use both lanes up to the merge, so that the traffic jam takes up less space. It might be the case that it's best to merge as soon as you figure out what's going on, to provide more time for the merge negotiation and reduce the risk of accidents at the merge point. In reality, it doesn't matter either way. If I try to use both lanes, I don't know that I'll be allowed to merge at the lane closure. If I merge early, I don't know if someone is going to use the now-unoccupied lane to zoom up and cut someone off, making both lanes slower and more accident-prone. I can't do either one and confidently say that I've made the correct choice, because I don't know how other drivers will react. What I do is just pick one, and slow down when something unexpected happens. This article also generalizes some things incorrectly (it seems based mainly on German data, presumably because they have better infrastructure for measuring such). For example, I live in the northeastern U.S. We are notoriously bad drivers up here, even among other east coasters. Here, a two-second following distance at 65MPH is almost unheard of -- actually, a two-second front-to-rear distance probably qualifies as a "large space", as some people here seem to think such a space can fit multiple vehicles. That's about 100ft or 30m, so I suppose it technically can, but it is not safe. I can react to this either by leaving a very large space (5-6 seconds seems sufficiently large that people get impatient and leave before it fills up) or by leaving a small space (.75-1.25 seconds is the largest I can leave and have a good chance of nobody trying to use it). The former is much safer, so that's what I do. The key is that, over a period of about two minutes, I need to maintain this following distance relative to the same vehicle. Usually this is fine: at most three cars tend to use that space at a time, and if anything they're more likely to tailgate the vehicle I'm following than to stay back near me. Occasionally I need to slow down in a situation where many vehicles fill that space, or someone moves in and immediately slows down. This is the exception, rather than the rule. Yes, I know this behavior can cause traffic jams under certain conditions and with certain other drivers -- but so could the alternative, under other conditions. Given the choice between two bad options, I optimize for my personal safety.
• Osiris 13y ago
  Has anyone tried to use genetic algorithms with traffic simulations to try to determine if there's a better way to manage traffic, especially in regards to the design of merge lanes, on/off ramps, etc? I've thought for a long time that it would be fascinating to build a complex traffic simulator that could be used by people designing freeways, neighbors, shopping malls, etc to plan for traffic. Right now those designs are just based on best guesses (or terrible guesses) by architects and not simulations.
• confluence 13y ago
  Smooth flow doesn't get you there any faster than highly variant turbulent flow. This is obvious. Now given that there will be little difference in overall throughput would you rather a) have a relaxing smooth drive or b) a stressful highly variant aggressive drive? The answer should be a, because the increased of risk of a fatal crash vs. the miniscule return of a few car lengths is not worth the effort you put in. You can verify this yourself. Drive as smooth as possible. Smoothly accelerate and decelerate. Leave large gaps. Try to use both your accelerator and brakes as little as possible. Now observe aggressive drivers and memorize their car's make, model and license plate. Observe the amount of risk and effort they undertake to get ahead. Then observe them only getting about 5-10 car lengths for their trouble. Finally observe them stopped behind you two traffic lights down. Laminar flow = turbulent flow, but laminar is so much better. This is besides the fact that laminar flow reduces traffic waves, reducing the variance for all those behind you, leading to a reduced risk of death and injury for everyone in your lane. My driving style has probably saved a life, or two.
• titaniu 13y ago
  How to avoid traffic: work from home.
• gqvijay 13y ago
  Hmm. Interesting. I have always contemplated this problem. First, I want to simplify the problem a bit and hope it'll apply to traffic jams. I think the problem is human reaction time. I must be missing something but hardly people talk about this. (although my solution might not be feasible). Let me explain. So, lets say you are stopped behind 30 cars and the first car is waiting for the red to turn green. Now, as soon as it turns green, it takes a second (or two) for the first car to start moving. After that second, second car adds another second before starting to move. Based on this, you won't move your car until 30 seconds have passed. Isn't this one of the big issue? If all of 50 cars instantly start driving once it hits green, wouldn't it reduce traffic jam greatly. Now, I do realize this is not feasible but if computers were controlling the car, 50th car can move instantly after green. On a side note, I realize the bottleneck situation can't be solved easily BUT again, I think human reaction (response) time is a huge part of it in most of these scenarios. If computers were driving, I wonder if we can have 10 times the car we have today without traffic jams.
• galaxyLogic 13y ago
  "... there’s not much benefit to trying to 'cancel out' a traffic wave by leaving a ton of space in front of you." What do you mean by a ton of space?. What do you mean by 'not much'?" The real question we should try to answer is "What is the optimum distance all cars should try to keep between them? And that can't be answered qualitatively, only quantitatively.