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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 leav

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.
  - hedora 7y ago
    Because you insist on the gap, your lane is 5mph slower than the other lane (because people pull in front of you after passing). Assume the other lane is doing 50 (65 mph speed limit, but there is congestion). That lowers the throughput (number of cars that complete a trip in your lane) by ~ 10%. Also, the faster lane is now probably moving closer to your speed behind you than the 50mph you observe (because you are causing cars going 45mph to cut off cars going 50mph that lane). The arrival rate of cars won’t change because you chose to go slow. So, if the road can handle 10,000 cars per unit time with normal congestion, it can now handle ~ 9000. That means per unit time, there are 1000 new cars sitting in the stop and go backlog that you single handedly created by creating the gap. This has been shown time and time by traffic simulations. It only takes a small handful of slow drivers to create massive backups. Also, the number of traffic fatalities is proportional to the difference in speed between the fastest and slowest car, so, statistically speaking, by being the slowest car, you are murdering people.
    - Noumenon72 7y ago
      The reason I am going 5mph slower in this thought experiment is because a continuous stream of cars is merging in front of me from the other lanes. (What the OP called the 'pathological case'.) They are the reason the lane is slower, and because they are not in the other lanes, those lanes will be 10% faster. With the pathological case being so benign, the real life experience is even better. You might get a car moving in front of you once a minute, and usually a few seconds of slower accelerating or coasting is all you need. To me the thing that really slows down traffic is people not moving left to let onramps merge. I think that's because they are afraid of missing their exit because no one is leaving gaps.
  - joshuak 7y ago
    I don't think you've considered what you are saying. You're not describing a gapping strategy, you're just describing a slower driver (i.e. slower traffic). There is no question that a car consistently driving slower than ambient traffic speed is a net drag on overall throughput. In any single lain of traffic all cars are bound by the slowest car. If you drive 5 mph slower than the car in front of you then the car behind you must drive 5 mph slower still to continually produce gaps behind you. As the video illustrates introducing a slower driver is one of the things that creates phantom stop signs. Of course you must maintain speed. The point of a gap is to allow for adjustments without the hard breaking that would create a phantom stop sign. That is the overall intent of the system. Move the most people as expediently as is safe from point a to point b. The best way to do this is to avoid speed differentials, being slower then ambient traffic speed creates a speed differential.
thrower123 7y ago
> 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. I just drove a Ford Edge that had this. It is a little overeager about what it thinks is an impending collision, but it works alright. The adaptive cruise control is a mess though. In theory, it is supposed to match your speed to the vehicle ahead if they are slower, so you don't have to pay attention. In practoce, it has the potential to be a great contributor to congestion. With dumb cruise control, you can push slowpokes to get out of the way or at least speed up; the adaptive cruise control seems to match speed at a distance where the driver ahead doesn't get the signal that they are obstructing things, and they continue bumbling along.

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.
- hedora 7y ago
  Because you insist on the gap, your lane is 5mph slower than the other lane (because people pull in front of you after passing). Assume the other lane is doing 50 (65 mph speed limit, but there is congestion). That lowers the throughput (number of cars that complete a trip in your lane) by ~ 10%. Also, the faster lane is now probably moving closer to your speed behind you than the 50mph you observe (because you are causing cars going 45mph to cut off cars going 50mph that lane). The arrival rate of cars won’t change because you chose to go slow. So, if the road can handle 10,000 cars per unit time with normal congestion, it can now handle ~ 9000. That means per unit time, there are 1000 new cars sitting in the stop and go backlog that you single handedly created by creating the gap. This has been shown time and time by traffic simulations. It only takes a small handful of slow drivers to create massive backups. Also, the number of traffic fatalities is proportional to the difference in speed between the fastest and slowest car, so, statistically speaking, by being the slowest car, you are murdering people.
  - Noumenon72 7y ago
    The reason I am going 5mph slower in this thought experiment is because a continuous stream of cars is merging in front of me from the other lanes. (What the OP called the 'pathological case'.) They are the reason the lane is slower, and because they are not in the other lanes, those lanes will be 10% faster. With the pathological case being so benign, the real life experience is even better. You might get a car moving in front of you once a minute, and usually a few seconds of slower accelerating or coasting is all you need. To me the thing that really slows down traffic is people not moving left to let onramps merge. I think that's because they are afraid of missing their exit because no one is leaving gaps.
- joshuak 7y ago
  I don't think you've considered what you are saying. You're not describing a gapping strategy, you're just describing a slower driver (i.e. slower traffic). There is no question that a car consistently driving slower than ambient traffic speed is a net drag on overall throughput. In any single lain of traffic all cars are bound by the slowest car. If you drive 5 mph slower than the car in front of you then the car behind you must drive 5 mph slower still to continually produce gaps behind you. As the video illustrates introducing a slower driver is one of the things that creates phantom stop signs. Of course you must maintain speed. The point of a gap is to allow for adjustments without the hard breaking that would create a phantom stop sign. That is the overall intent of the system. Move the most people as expediently as is safe from point a to point b. The best way to do this is to avoid speed differentials, being slower then ambient traffic speed creates a speed differential.

thrower123 7y ago

> 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. I just drove a Ford Edge that had this. It is a little overeager about what it thinks is an impending collision, but it works alright. The adaptive cruise control is a mess though. In theory, it is supposed to match your speed to the vehicle ahead if they are slower, so you don't have to pay attention. In practoce, it has the potential to be a great contributor to congestion. With dumb cruise control, you can push slowpokes to get out of the way or at least speed up; the adaptive cruise control seems to match speed at a distance where the driver ahead doesn't get the signal that they are obstructing things, and they continue bumbling along.