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How traffic actually works

jliszka.github.io

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Re: How traffic actually works

#171

Post by dclowd9901 »

sk5t wrote: post #97
GigabyteCoin wrote: post #80

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 c…

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 sw…

It'll be really interesting to see how driverless cars negotiate things like natural merges (a freeway losing a lane), and how that affects traffic flow. Ostensibly they'll be as efficient as possible, but if two lanes become one, there's going to be some slowing regardless. Furthermore, in a vehicle that also ostensibly wants to get you to your point of interest in as timely a manner as possible, how does it decide whether to subject itself to staying in a right lane where it might be subject to moreane merges, or does it ever decide to go around a particularly bad or congested spot for the sake of getting you to the destination?

Re: How traffic actually works

#172

Post by rdmcfee »

msandford wrote: post #169
mavhc wrote: post #157

Earlier quoted context omitted.

That doesn't matter though, as if they're going faster they're more spaced out. Only 2000 cars per hour can pass the merge point

Sure, but when traffic jams happen you often see not just ONE lane of traffic slowing down at the merge point but ALL lanes of traffic slowing down. On a four lane freeway narrowing to three lanes it's not the case that the two right lanes are stop & go and the two left lanes are 80MPH. EVERYONE slows to 10MPH. Furthermore the notion that you'll get 2000 cars per hour at the merge point irrespective of speed is ludic…

It's not about velocity it's about rate of cars through a point per hour. If you have a mergepoint with 10 lanes. Each lane can support say 20 cars per minute. If you have 200 cars approaching the merge-point per minute the cars can travel effectively at the speed of their choice.

If you close one lane, reducing the capacity of the mergepoint to 180 cars per minute while 200 cars are approaching a queue will build. The speed with which the cars mass the mergepoint is not relevant because the rate of cars per lane per minute will stay basically at 20 cars/lane/min.

As to your point about all lanes slowing down - cars will always redistribute as you can imagine. People tend to merge left as there's an additional traffic stream merging on their right. The writer made points about the capacity of the mergepoint vs. the cars approaching- not individual lanes and speeds.

Re: How traffic actually works

#173

Post by teddyh »

susi22 wrote: post #154
teddyh wrote: post #123

Earlier quoted context omitted.

> You can see on the right side how his "cars per second" is _exactly the same_ than on the left side. Is it my browser, or is this completely wrong? For me, the right side moves about double the amount of cars into the upper edge of the image. And I believe that this is the point of the section — it wouldn’t make any sense otherwise.

Cars per second are the same. Open the two gifs next to each other. Each has 8 frames and the top right lamp is on for 4 and off for 4.

The lamp is irrelevant. Look at the cars.

Re: How traffic actually works

#174

Post by khuey »

interurban wrote: post #130
colanderman wrote: post #91

Earlier quoted context omitted.

There are highways in the US that do this with dividers. A specialized vehicle drives over them and shifts them one lane to the left/right twice each day.

A good example would be the Golden Gate Bridge (at least they used to do this, I haven't been to the west coast in several years).

Still does, but it's only over the bridge. The approaches have fixed bandwidth and can back up.

Re: How traffic actually works

#175

Post by mavhc »

rdmcfee wrote: post #172
msandford wrote: post #169

Earlier quoted context omitted.

Sure, but when traffic jams happen you often see not just ONE lane of traffic slowing down at the merge point but ALL lanes of traffic slowing down. On a four lane freeway narrowing to three lanes it's not the case that the two right lanes are stop & go and the two left lanes are 80MPH. EVERYONE slows to 10MPH. Furthermore the notion that you'll get 2000 cars per hour at the merge point irrespective of speed is ludic…

It's not about velocity it's about rate of cars through a point per hour. If you have a mergepoint with 10 lanes. Each lane can support say 20 cars per minute. If you have 200 cars approaching the merge-point per minute the cars can travel effectively at the speed of their choice. If you close one lane, reducing the capacity of the mergepoint to 180 cars per minute while 200 cars are approaching a queue will build. T…

Indeed. The more interesting points to be discussed are

1. If the flow rate changes with speed, thus merging at 5 mph instead of 50 is bad.

2. What will prevent accidents. I suggest start-stop traffic causes more accidents than free flow.

3. Fuel efficiency, which would depend on many things, eg the percentage of cars in traffic that stop their engines when the car is stopped.

Re: How traffic actually works

#176

Post by crististm »

analyst74 wrote: post #104
susi22 wrote: post #86

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 eve…

Having smoother traffic is good on many grounds: happier drivers, better fuel efficiency, and what not. But it does not make people go home faster. Let's put a simple deductive thinking practice: If you are the first car in the jam, you are stopped and slowed by traffic lights, police, accidents or whatever, but the traffic jam does not affect you, you are going as fast as you can. For the second car, you will not ge…

Yes it does get people home faster if only because there is no latency on speeding up after breaking.

Re: How traffic actually works

#177

Post by James_Duval »

peterwwillis wrote: post #114
susi22 wrote: post #86

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 eve…

Leaving space does not increase the average speed, because the average speed is dictated by the MAXIMUM speed in addition to the number of cars on the road. -- First we must assume we live in the 3rd dimension of a physical realm, wherein objects like cars cannot pass through other objects. Cars can not drive through other cars, thus, if one car stops or slows down, all cars behind it stop or slow down. Second, 5000…

There is a huge benefit related to your point about smoothing out human error:

People who pick the wrong lanes can change back immediately and travel the right way rather than either (a) slowing to a crawl with their indicator on hoping to be allowed into the correct lane by some kind soul (who also has to stop/slow down) or (b) looping back round on themselves and trying again.

Obviously, both of the latter solutions add to congestion, and the most common solution I see is an awkward combination of both (a) and (b).

EDIT: In addition, smoother-flowing traffic is desirable in-and-of itself, as fuel economy is greatly improved.

Re: How traffic actually works

#178

Post by Cthulhu_ »

crististm wrote: post #176
analyst74 wrote: post #104

Earlier quoted context omitted.

Having smoother traffic is good on many grounds: happier drivers, better fuel efficiency, and what not. But it does not make people go home faster. Let's put a simple deductive thinking practice: If you are the first car in the jam, you are stopped and slowed by traffic lights, police, accidents or whatever, but the traffic jam does not affect you, you are going as fast as you can. For the second car, you will not ge…

Yes it does get people home faster if only because there is no latency on speeding up after breaking.

This. Smooth driving will make for a higher average / continuous speed, not to mention it'll cause less wear (brakes, clutch) and lower fuel consumption (accellerating costs fuel, braking is a waste of motion energy)

Re: How traffic actually works

#179

Post by sirsar »

SilasX wrote: post #137
chrisa wrote: post #11

Earlier quoted context omitted.

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...

"Yeah, so, it turns out that charging for something prevents its overuse." 'WOW! REALLY?'

I think the point is the nonlinearity of speed as a function of the number of cars, i.e.,

"Charging for something reduced use by 20%, and overuse-related problems by 95%"

"WOW! Why don't more people do that?"

Re: How traffic actually works

#180

Post by privong »

methodin wrote: post #143
privong wrote: post #73

Earlier quoted context omitted.

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 d…

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"…

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.
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