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How traffic actually works

jliszka.github.io

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Re: How traffic actually works

#11

Post by chrisa »

bryanlarsen wrote: post #5

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.

This doesn't sound like the same study, but it's similar: a TED talk about how adding a fee for bridges in Stockholm reduced the number of cars by 20% and effectively eliminated congestion: http://www.ted.com/talks/jonas_eliasson_how_to_solve_traffic...

Re: How traffic actually works

#12

Post by tucaz »

hammock wrote: post #2

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 import…

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.

Re: How traffic actually works

#13

Post by protonfish »

Dirlewanger wrote: post #7
hammock wrote: post #2

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 import…

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 e…

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.

Re: How traffic actually works

#14

Post by josefresco »

jliszka wrote: post #4
themodelplumber wrote: post #3

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?

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.

Doesn't this simply displace some of the wait to the onramp?

Re: How traffic actually works

#15

Post by petenixey »

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.

Re: How traffic actually works

#16

Post by exue »

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-... http://www.dot.state.mn.us/trafficeng/workzone/doc/When-late...

Re: How traffic actually works

#17

Post by jameshart »

daniel_levine wrote: post #8
bryanlarsen wrote: post #5

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.

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 muc…

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/A...

Re: How traffic actually works

#18

Post by meatwad »

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?

Re: How traffic actually works

#19

Post by methodin »

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.

Re: How traffic actually works

#20

Post by cecilpl »

tucaz wrote: post #12
hammock wrote: post #2

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 import…

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.

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.

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