What Are Traffic Waves and Why Do They Happen So Much?
ww2.kqed.org
ww2.kqed.org
It discussed traffic waves and our reactions to them. The repeatable waves this article mentions come from (most of) the individual drivers driving faster than the capacity of the road to carry them all. So they bunch up at some point, catching those in front, everyone slows down for a bit and then the bunch "evaporates" from the front... repeat. Those who get out of the slowdown speed to the next one. It also discussed the effects of merging, which have a high cost as lanes get fuller, also why you'll see lights at on-ramps to mitigate.
The recommendation was to slow down, leave plenty of space ahead for others to merge, and try to get going at as close to a single speed as possible to conserve momentum, gas, wear & tear, nerves, etc. If packed enough you can let the car idle push you along around 5mph/8kph, which is what I aim for in that situation. Feels better than speed-up/break/slow-down over and over.
It also responded to questions such as, "what if everyone around me wants me to go faster?" It happens, but those people tend to drive off in a huff. Once one or two of them do, you'll find a more relaxed driver behind you for the rest of the trip. The impatient driver won't get ahead much anyway so you can wink at them when you catch up at the next slowdown.
I'd like to avoid the wear on my car and my nerves, but it's not possible because some (expletive) person will fill the gap.
(or was I being trolled? see "comedy in Boston" in one of the "cousin" comments nearby)
http://trafficwaves.org/trafexp.html
If you arrive any later it will be minute or two per hour, so worrying about people "getting ahead" of you accomplishes nothing. If you want to win the race against the others on the freeway tomorrow, leave 5 minutes earlier!
I read it too, years ago, and also follow its recommendations. (And I ended up in the Seattle area, so I've gotten to experience the same traffic the author did...) It makes traffic less annoying for sure, even if there were 0 other benefits I'd do it. The only freeway issue that annoys me these days is people who don't keep right except to pass during non peak times.
In reality it could be just the opposite, someone behind you could be blindly accelerating because he sees the cars next to him doing so, then realizes you in front of him aren't accelerating along with them and has to tap his brakes, causing more traffic waves. And by definition the traffic behind you is more compressed than it otherwise would be, because you're hogging tons of lead space all for yourself. So you're dampening the waves in front of you by making the waves that develop behind you even worse!
Given that the minimum amount of lead time drivers are comfortable with is constant, if you pour tons of cars into a highway at rush hour, traffic necessarily must slow down because there's too many cars per mile of road to accommodate a safe lead time for each car at high speed.
So if everyone tried to smooth out traffic waves by leaving extra space in front of them, they'd effectively be making the number of cars that will fit in a mile of road even lower by artificially requiring more space for themselves. This will always make traffic become slower overall. Say what you want about the benefits of not having to stop-and-go traffic but you shouldn't be under any illusion that it makes the commute faster overall.
Person 1: You can fix traffic waves by smoothing!
Person 2: Smoothing traffic waves makes no sense!
It's frustrating. I don't know what to believe. Is there any peer-reviewed, simulation- and data-backed research that puts this issue to bed for good?
Given that everyone requires a safe lead time of (for example) 1-2 seconds, the more cars there are per mile of highway, the slower everyone must drive. (You can't drive 80 miles per hour bumper-to-bumper.) So if density is say, 50% (one carlength of open space per one car), you have to drive a speed such that a carlength is 1-2s, in other words 10-20mph.
But traffic distribution is not uniform, there's exits and entrances, and cars do occasionally need to change lanes. If traffic is going 10-20mph with 1 carlength of space between cars (steady state), and I change lanes, the guy I merged in front of now has to slow down more to leave more room, and this will cause a traffic wave behind him. What happens at an exit when half the cars change lanes? Standstill. No change in driving technique on anyone's part will help this.
I think the only times where the way you drive matters is when the density is kinda sorta high but still low enough for a safe following distance at reasonable speeds, at which point "smoothing out waves" becomes a common sense matter of "don't follow so close", which is effectively the same thing, and something everyone should be doing anyway when the density is low.
A lot of traffic problems stem from the fact that you get more cars per second from quickly moving smooth traffic than from slowly moving dense traffic. Therefore if something bad happens to rapid smooth traffic, you quickly get a phase transition into horrible traffic.
Now what does the transition from horrible traffic back to good traffic look like? The first step is that you have to get back to smooth traffic, and then that has to speed up.
If nobody smooths out traffic deliberately, then you don't get this until the volume of people wanting to pass gets so light that all of the stop spots "evaporate" on their own because there is no pressure on them. Then the road speeds up. By contrast if traffic has been smoothed, the bottom speed steadily increases, and you get back to full speed much earlier.
Plus smoothing traffic will save on your own car's wear and tear.
Oh, and on merging? Go read http://ops.fhwa.dot.gov/wz/workshops/accessible/McCoy.htm. You will find that in the real world when drivers are instructed to merge late and politiely, merging happens significantly faster, with far fewer conflicts between drivers. So how you merge actually does matter. A lot.
Even the wikipedia article on "Traffic flow" is quite comprehensive. [0]
It really annoys me when people come up with a "theory" and write a blog post about it without doing a shred of actual research on the subject.
The big benefit, which the grandparent concedes, is the reduction of unnecessary fuel usage, wear and tear on the vehicle and driver.
So, I consider it a win regardless.
In theory the waves can represent a bottleneck, since the close-packed regions are low-flow, but the wide empty regions are also low-flow. If the smoothed wave results in traffic flowing at 35-40MPH, then smoothing will increase the flow, since the peak flow rate takes place at 35-40 MPH. But the increase isn't enormous. I think they said ~15%. That's nothing, when compared to the effects of removing an accident scene from a rush-hour commute.
You can play with those online JAVA simulators: set up unstable conditions on the ring-road, watch the initial flow, then after traffic-waves develop, watch the flow again. If enormous waves appear, the flow drops by ~25%, but if a string of small waves appear, the decrease is less than 10%.
Also, I didn't say that smoothing the waves increased road capacity, though improving merging may help a bit. Rather, the reduction of stop/go is to everyone's benefit.
In the city things are largely the same, people here tend to speed to the next red light, but it doesn't help.
Because, uh, you don't drive slow.
You drive at the average speed of traffic. Not slow. To smooth out the traffic waves, you don't come to a halt, and neither do you rush forward. "Average speed:" that's when it's not fast, and also it's not slow.
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Myth: if you have an empty gap, you must be a slow driver!!
Debunked: yes, genuinely slow drivers have big gaps ahead, but those gaps are continuously growing. If a clot of cars is going just 1/2MPH slower than average, then the gap increases rapidly: growing a half mile for each driving hour. If that clot of cars was 5MPH slow, then in just ten minutes the gap would increase by over four thousand ft.
But on the other hand, a constant-sized gap does not decrease your speed. Whether you're 6" from the car ahead, or 6ft, or 600ft, your gap has a constant size, and you're moving at the same speed as traffic ahead.
Slow drivers do make empty gaps, but empty gaps aren't any proof of slow driving.
Heh, so maniacal aggressive tailgating doesn't actually get you to your destination any faster? AMAZING! Who'd have thought!
The movie portrays it as conversational kryptonite, where he briefly discusses traffic issues in order to weaponize boredom and get some people to leave him alone.
That part broke my suspension of disbelief, because I was like "Hey, why is everyone wandering away?!? I want to hear what Tom has to say about how to address the problem!"
If a highway has two lanes, the average speed in the left lane is higher than the average speed in the right, but the only major passing opportunities are on the right when there's an empty stretch (because no one merges right).
If a highway has >2 lanes, the left is never consistently the fastest moving lane, and almost every passing opportunity comes from moving right. It is not uncommon to find the right lane near empty, and it is then possible to blow by everyone in that lane.
The exception to the above is in areas with heavy merging, wherein the average American driver seems to not understand how to merge (either blocking one another, or coming to a stop as the merge lane begins). In this case, speed seems to increase moving left.
I, too, have noticed that people here tend to just pick a lane (usually the left one to avoid trucks) and stick to it no matter the speed they're going. However, my experience in the other two states I've lived in (Florida, Indiana) and the other 35+ states I've visited, is that most people in the US know that the left lane is for passing and are somewhat more considerate about this.
Drivers have trouble merging in all 50 states, though. Half the people merge as soon as possible, thinking it's more polite to merge sooner rather than later (or just not wanting to fight for an opening or get angry looks from people who believe merging early is better), and half the people merge as late as possible, attempting to do the zipper merge which is actually the most efficient means of merging and what everyone should do.
Autos with trailers and trucks (vehicles subject to the 55mph limit where other vehicles have a 65 or 70mph limit) are not generally restricted to the right lane in California.
They only are restricted to the rightmost lane on roads where: (1) where there is no other designated (by sign) set of lanes for such vehicles, and (2) The road is not a divided highway with four or more lanes in the direction of travel (where the default, without a designation, is that they are restricted to the two rightmost lanes for normal traffic, plus the next lane out for passing.)
On a road with 1 lane per direction, trucks being restricted to the right lane except for passing has no negative impact on your preferred practice regarding use of the left lane in multilane roads.
On a road with 2 lanes per direction, trucks being restricted to the right lane except for passing exactly matches your preferred practice, though the lower truck speed limit (independent of any lane restrictions) might arguably encourage other drivers to stick in the far left lane (though, except in areas with high density of trucks, doesn't really provide any strong incentive not to follow the use the right lane for traffic, except for passing, it just increases the likelihood that non-truck traffic will want to pass trucks, which is pretty high in any case.)
The lane discipline which doesn't apply on roads with 4+ lanes in one direction thus is really even potentially even a factor in what you are concerned with on roads with exactly 3 lanes per direction.
I've started calling the first merge style you've observed the "Minnesota merge". I deal with it on the way to and from the office, each direction with a good mile of merge lane.
Ninja Edit: Jersey is the best for clear right lanes. I've made entire journeys never coming up on anyone in the right lane, unless they were in the process of "merging", which in this case means getting all the way into the left lane immediately.
I remember being taught this when I started driving in the 90's. And I remember people following it until the mid/late 2000's. It's as if a switch was flipped, and nobody cared about this practice anymore.
I think it's an attitude thing. As often as I see drivers in my area blowing through red lights and pulling other ridiculous or dangerous stunts every single day, I can only imagine it's due to a lack of care or respect on the drivers' part.
The reality is that cars are becoming very ineffcient methods of transport. Most carry only one driver, take up a lot of space, etc. The days of driving on a rural highway with two lanes and having the left lane designated as just the passing lane are long behind us. We don't have the capacity for that outside of rural areas and as more areas become congested with car traffic, the less polite people will be.
It includes drivers with different personalities (buses accelerate slower, some drivers are daydreaming, etc.) and lane changing (people change lanes when it's beneficial)
In which countries did it happen more?
It does not take many drivers acting responsibly to fix a wave jam. Interestingly, culturally, is that this responsible action is viewed here as a relaxed take on traffic. When the line restarts, some drivers take their time shifting into first gear, and then take a lot of time to increase their speed. They effectively buffer cars behind them from the stop-and-go.
If you get two of these in a row, it's enough for the stop and go effect to dissipate.
If you do this on the road, prepare to be overtaken by someone "in a rush", and prepare to ignore the action, although it's acceptable to gesticulate and swear at the car ahead :-)
I'm not sure what you're trying to imply there, but it seems a bit racist? Or am I just being ignorant?
I'm not sure if a model exists yet for cars 'putting out' traffic waves, but I'm sure once we have self driving cars and they start talking to each other this is going to be of great interest.
I'm hoping we can effectively 'micro' away some degree of congestion.
In the end, I think most of the communication will just be in terms of the fact that self-driving cars interacting with other self-driving cars will be much smoother and more normal than existing traffic.
Look at flocking birds. They're not talking to each other, really. They're reacting in a complex but uniform way to their peers. I suspect self-driving cars will look like that even without active communication.
Is 'no signal' the same as 'no other cars' or 'broken radio(s)'?
Self driving cars have all the elements of a distributed computing system only now it's moving at a pretty good clip and lives are at stake. 'Move fast and break something' is very bad advice and 99.999% uptime is not going to cut it.
Hard enough to get autonomous (pun not intended) systems right, much harder to make systems that communicate between vehicles right. The best way would be to not rely on inter-vehicle communication but to only work with the signals that the car can pick up from the environment.
At least that data will be somewhat trustworthy because you have some idea about the integrity of the system.
You certainly wouldn't depend on communication for collision avoidance, but it's still likely that cars will have beacons which alert other cars to their existence, and it has nothing to do with -- at the range of kilometers -- taking turns broadcasting how many cars you see around you (immediately, ie, traffic packing), your velocity, and your location.
The other self-driving cars can listen for beacons about congestion, and adjust strategic parameters (such as speed) in a safe manner. While I wouldn't trust a random driver telling me to break suddenly, why not take a random tip that there's a slowdown and adjust my driving to help mitigate the jam, especially if 50 other people agree that there's a jam there?
In the long run, I expect cars to become smart enough to be self-owning, in which case it's every car for himself.
If we get sufficiently far out (decades in, after human-driven cars are completely gone, not just out of obsolescence, but due to being outlawed at least for certain areas), I'd bet that what we'd see are demands for compliance that resemble the restrictions on automakers that we have now. I imagine that the driving programs would eventually be in the hands of some regulatory agency, with municipalities being allowed some control of the parameters.
They didn't need to back off, and in that case, you would have just waited for an opportunity to open up, or a friendlier driver to be next to you. Even if they nodded, but didn't back off, you'd just assume a mis-communication and not try to merge in.
So, self driving cars could do this on a much more regular and reliable basis, while still not relying on it or trusting one another. They broadcast that they want to get over, and if another car happens to get that signal and happens to be willing/able to comply, it opens a space. If the first car sees a space, it moves over, which is what it would have done anyway, regardless of whether the space is coincidental or intentional.
Could also be handy for determining right of way in edge cases where humans often get confused and play the you-go-no-I-guess-I'll-go-no-wait-now-he's-going game. Two computers might be playing the same game, but they can resolve it in a tiny fraction of a second. Of course, the flocking birds example works well here as well. If the rules are clear enough, the cars will just know who should go.
But yes, no matter what, a car couldn't trust what another will do, and should be prepared with an evasion strategy for every possibility, thus not doing anything that would leave it no way out if another car did something stupid or there was a mis-communication.
If the vehicle in front of you tells you it will accelerate, you shouldn't accelerate too, unless you can verify the acceleration and that it's safe for you to do so.
http://journals.aps.org/pre/abstract/10.1103/PhysRevE.69.066...
So if just 20% of humans were safe drivers (spaced out properly,) then everything smooths out.
That, or perhaps 0.5% of drivers could intentionally go at the average speed.
This often upsets people when they get out of a jam only to see there was nothing causing it! In reality what caused it may have been much further up the road.
But honestly, how does this reduce congestion? My experience is that these motorcycles often dart between cars causing drivers to swerve slightly and tap their brakes.
I would argue this makes congestion worse as it causes more cautious drivers to brake (sometimes abruptly) and almost always slow down.
Finally, it is important to consider that as a vehicle, most of us are not looking for you and make small changes from left to right while in our lanes. It is not a goal of mine to accommodate motorcycle riders while driving. Splitter beware.
I'm all for sharing the road-- be we all need to subject to the same rules.
Lane splitting is only supposed to occur below a certain speed; splitting above that number would be a violation, of course.
Take a look at Europe to see how easily splitting can be integrated, for both driver and rider. It absolutely does reduce congestion: bikes move smoothly to the front and away, freeing space.
American drivers just aren't educated to realize this because negative stereotypes about motorcycles are so prevalent in the US, and the many positive benefits of riding aren't presented as options here.
1. Divided lanes for 3+ lane highways. Basically a modified HOV lane but no occupancy restriction. The far left lane would have lane dividers with openings every 5 miles (or 3 exits). The middle lane opening every 2-3 miles (or every other exit). The idea is that it would force a certain average speed of traffic and hopefully distance between cars in the respective lanes.
2. Smart roads to adequately distance cars from each other. Either a driver assistance signal or actually controlling the internal cruise control system.
It's fun to see children (and adults!) play with the controls on this model, experimenting with different factors that they think might eventually reduce the congestion.
This model is also a great way to introduce kids to complex systems, in which the large-scale results aren't necessarily as obvious as the individual actions that lead to those results.
I'm not sure how well most users understand how it's supposed to help them go faster by slowing down; a bit more education could be helpful so everyone plays the game.
It is a bit weird seeing the speed limit jump from 45 to 65 then to 35 within the space of a mile…
[1] http://www.investopedia.com/articles/technical/111401.asp
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