Experiment Shows Traffic 'Shock Waves' Cause Jams

Galactic_grub writes "Japanese researchers recently performed the first experimental demonstration of a phenomenon that causes a busy freeway to inexplicably grind to a halt. A team from Nagoya University in Japan had volunteers drive cars around a small circular track and monitored the way 'shockwaves' — caused when one driver brakes — are sent back to other cars, caused jams to occur. Drivers were asked to travel at 30 kmph but small fluctuations soon appeared, eventually causing several vehicles to stop completely. Understanding the phenomenon could help devise ways to avoid the problem. As one researcher comments: 'If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.'"

Brakes. Not breaks.

[caluml (551744)]

1. It's brakes. Brakes. Breaks is when something stops working. 2. This is obvious to anyone who has driven much. Try not to use your *brakes* on the motorway. Try to "iron out" the waves by ever so slowly dropping back when you see them approaching.

Re:Brakes. Not breaks.

[wattrlz (1162603)]

Well, it's an understandable mistake. They're, "Breaking" the flow of traffic, after all.

Not that simple

[Gription (1006467)]

I really hope that this isn't truly a "new" discovery. If someone sits and thinks about it it should be really obvious. I have posted a basic explanation as a comment to a number of blogs and I'm not a traffic engineer.

Once traffic reaches a point of saturation ANY change is flow will bring traffic to a halt. This includes things like a road feature that makes people accelerate, a merging or diverging ramp, or a hill or corner that allows people to see farther ahead inducing them to adjust their speed.

Any change in relative speed will at some point cause a person to slow down. If they are accelerating they will slow when they get close to the car in front. If they are slowing then that is obviously a slow down. Each car behind this change reacts to it after a delay. The longer the delay the more they have to react. People tend to react much slower then they normally would because very few drivers focus primarily on driving. They only use the minimum attention that is necessary. This just makes the problem worse. It only takes a few cars for a subtle adjustment to become a complete stop. The length of time each successive car is stopped will become longer and longer.

To make the problem worse the same 1 to 2 second delay in reaction applies to the acceleration of the vehicles after the slowdown/stop. Every fraction of a second that a person delays accelerating is adding to the stacking of the delay.

To look at another aspect of a slowdown...
If you have a smooth flow of traffic at a fixed speed you will have a certain number of cars passing a point on the road per minute. If you have a slowdown you will reduce the number of cars passing per minute. But the traffic was already at a capacity flow so the number of cars feeding into this situation continues unabated. It is obvious that the flow of traffic is done until the quantity of vehicles feeding into the situation is drastically slowed to less then the flow of traffic at the restriction. (In Orange County, CA that means waiting till after 6:30PM for a situation that started at 3:00PM.)

What can be done about this without building a vastly increased number of lanes? (The I-405 is already 14 lanes in central Orange County. Increasing it to 20+ lanes would cost billions and in 20 years when they are done we would need 30+ lanes...)

A big improvement could be made through driver training (Yeah right...). Teaching people how to merge can reduce the constant forced slowdown from cars merging onto a freeway at less then freeway speed. The correct speed to merge at is 5 to 10 MPH FASTER then the flow of traffic. A car won't accelerate quickly but it will slowdown quickly so you simply drive down the on ramp picking the hole in traffic you will merge into and brake into that hole matching speeds. This minimizes the disruption. "Freeway Meters" on an onramp actually make this problem worse on a freeway that is still flowing at a reasonable speed because they reduce the distance that a car has to accelerate insuring that they enter the roadway at a reduced speed causing traffic to slow down for them.

Teaching people to slowdown after they are on an offramp will also help reduce the disruption to traffic flow. Most drivers slow down a minimum of 10mph before the exit the roadway causing large backups for open free flowing offramps.

To reduce the effect of slowdowns you can teach people to look past the car in front of them and try to slowdown before the car in front of them, which can reduce the quantity of speed they have to scrub off. If they look ahead and start to accelerate earlier this will cause a similar improvement in reduction of traffic impediment.

(Truth is that you will never be able to teach people to change their driving habits because their cell phone, their coffee, and their daydreams are way more important then driving the 1-1/2 tons of steel they are sitting in.)

Re:Not that simple

[cbiltcliffe (186293)]

I really hope that this isn't truly a "new" discovery.

It's not. This guy [eskimo.com] was an amateur looking at the problem a decade ago.

Re:Not that simple

[internewt (640704)]

Its always a BMW because they think they own the fucking roads. They also rarely indicate, and if they do it'll be incorrectly.

Mercedes drivers are awful too, but has anyone noticed just how much of a wanker the average Audi driver seems to be these days? Many car buyers are fashion victims, and I think some of the "cooler"[1] BMW drivers are moving to Audis.... I don't think its a co-incidence that the newer Audis look more aggressive than the BMWs.

[1] shallower or wankerer

Re:Not that simple

[jonbryce (703250)]

Maybe you should visit the M42 in Birmingham, England, where a trial of allowing people to use the hard shoulder at peak times was considered a success, and it is going to be rolled out across the rest of the country.

What happens if there is a break down? You are back to three lanes of traffic, just like there would have been if you had never allowed driving on the hard shoulder in the first place. The rest of the time, you get an extra lane.

Re:Not that simple

[omeomi (675045)]

I really hope that this isn't truly a "new" discovery. If someone sits and thinks about it it should be really obvious. I have posted a basic explanation as a comment to a number of blogs and I'm not a traffic engineer.

I think what's new here is that it's been shown in an actual experiment using real cars, rather than just theorized or modeled in a computer.

If more is better then too much is just right...

[Gription (1006467)]

You bring up an issue that connects with fuel consumption and safety on the road.

It is pretty rare that anyone truly questions the vehicles that we buy today. People have accepted SUVs as the norm for driving around in the US for some reason. People will recite a number of reasons but in reality the only real reason is that they have become accepted as normal and 'cool'. It is just a change in peoples perception. If you had asked someone in the 1970s to select a 'people hauler' they would select a station wagon. It is lower to the ground making it safer to drive and is a much better layout for a vehicle. No one would have accepted a 70s Suburban, Blazer, or Bronco (even with a more luxurious interior) because of the perception, "That's a truck!"

This change of perception is actively damaging our country. Moving the increased mass of these vehicles around just plain costs energy and increases wear and tear on our roadways. If you are conscious of this and want to get a reduced mass commuter vehicle you are taking your life in your hands because of the battering ram reality of a large percentage of the vehicles.

It is really time that we do something realistic about the mass of the vehicles on our roads. A general switch to smaller lightweight vehicles would massively reduce fuel expenditures, pollution, and the smaller size would help to reduce congestion. The solution to this is to change public perception, which will probably only come about by economic reasons. The price of gas will do part of it but taxing vehicles by weight will go a long way towards making it a more equitable situation. The lighter vehicles would be rewarded for their lessened impact on the roadway and the environment. More massive vehicles would pay for increased impact on the environment, wear to the roadway, and the increased risk they pose to lighter vehicles.

People talk about rail/mass transit as a solution to LA's transportation problems but usually it is people who haven't been here. This area is so spread out in all directions that it would take an indescribable quantity of money to build such a system. The land is insanely expensive and with the sprawling area you could spend a few years of our whole nation's federal budget to build such a system. It just won't work.

(That guy who stole the tank down in San Diego about 5 years ago is starting to look saner and saner...)

BTW - From trips to Europe: I love the mass transit! Munich has a rail, subway, tram, and bus system that is so good it is just fun to use it! For 15 euro you get a pass good for unlimited use of all of the above for a week! It is just amazingly convenient and cool.

Re:Brakes. Not breaks.

[Yold (473518)]

Tailgating is a problem too. It really pisses me off, that even in non-rushhour traffic, some idiot is always less than a car-length off my back end. Leaving a buffer zone allows you to avoid using your breaks when traffic slows.

I wonder how much aggressive driving (someone speeding up to 90, and then cutting in front of you for seemingly no reason), contributes to breaking shock waves. I've seen it happen often enough where someone will make an unnecessary maneuver to get 30 feet ahead of traffic.

Re:Brakes. Not breaks.

[Strawser (22927)]

It's a combination of people who ride slow in the left-hand lane and speeders. The people (whom I'd like to shoot) that pull over to the passing lane and then drive the same speed as the car to their right cause rolling road-blocks. When faster-moving traffic inevitably catches up to them, it can't pass, so it builds up into a massive pack of slow-moving crap. Then, sooner or later, someone taps his brakes, and then the one behind him does it just a bit longer, and so on and so forth, until there's a stop for no reason. Meanwhile, the jerk-off in the left hand lane at the head of this rolling traffic jam is still doing just fine at 50 MPH.

If police would enforce rules against driving too slowly (generally defined as being passed on the right (because if traffic is passing you on the right, then you need to get the fuck over)) as they do aggressive driving, the problem would be much less prevalent. Then, the faster moving traffic could pass the slower moving traffic, keep on going, and there wouldn't be any problems. Sadly, though, that's not the case in any metro-area I've dealt with. Instead, the jerkoff Sunday Driver creeping along at 50 in the passing lane just has to be dealt with, usually by getting around him in the right hand lane, then speeding up to 90 and cutting in front of him so you can pass the traffic on the right.

Re:Brakes. Not breaks.

[Deadstick (535032)]

There's nothing you can do about this.

Maybe not, but there's something your highway authority can do about it: Adopt German rules. Passing on the right gets you a ticket; driving on the left without passing gets you a ticket.

Flipping someone off gets you a ticket too, but that's another issue.

rj

Re:Brakes. Not breaks.

[Dread_ed (260158)]

Your self righteous attitude is the cause of hundreds of lost hours of time for people all over the world. Instead of trying to be a one-sided-Buddha, teaching everyone patience, why not express some compassion and move over for faster moving traffic. Plus, if you are as patient as you wish other people to be you won't mind hopping into the slow lane for a few seconds.

Of course if you are motivated by some other less honorable sentiments you will continue to act as you do and feel justified and prideful about it.

Also, be aware that there are varying circumstances that may cause someone to drive faster than you. Your statement that it is because of some dissolusioned consumer fantasy belies your presupposition that the people who drive faster than you are somehow inferior to you. Believing a lie and then becoming irrationally angry with you because you show them the truth of the world makes an easy straw man to flatten. However, consider that you might be stopping someone from reaching an important destination in an appointed time. Like the time my father was in the hospital. His respiratory doctor called me and said he was not breathing without assistance and that I had very little time to get there to see him before the DNR orders took effect. I was across town at work and had no idea if I was going to get to see my father alive ever again. I cannot tell you how frustrating it was to deal with people *just* *like* *you* as I made my way to see if my father was still alive, to talk to me one last time.

But hey, one thing I have learned is that people act as they feel they must. Compelled is a good word. People who drive faster than me, I move over for them. People who drive slower than me, I just wait till I can pass them. Understanding that they have their own internal motivations that I probably do not understand makes it easier to live and let live. When I catch myself assigning characteristics, thoughts, and motivations to people driving near me I just take a deep breath and realize that I don't know why they are doing what they are doing and it is best to work cooperatively with people, even if I have never met them.

Re:Brakes. Not breaks.

[sm62704 (957197)]

Tailgating is THE problem.

I try to stay far enough behind the car ahead of me so that when he brakes, I merely have to remove my foot from the accelerator so I don't convert my kinetic energy to heat. Of course, some dipshit always sees the three car length hole in the thirty mph traffic (you're supposed to maintain 1 car length for every 10mph anyway but none of the fucktards in Springfield do) and fills it in.

If people maintained a reaonable distance (the 1 car lenhgth for each 10 mph) you wouldn't have this effect, or if it occurred it wouldn't be so bad.

Every time you touch your brake for any reason whatever you throw fuel away as waste heat.

<jk>(global warming comes from the hot air blowing out of the world's capitols)</jk>

-mcgrew

Re:Brakes. Not breaks.

[ectal (949842)]

I like the 3 second rule better than the car length rule. Regardless of your speed you should be 2-3 seconds behind the next car.

And yes. Tailgating certainly is a HUGE problem. What's most astonishing to me is how many people shrug it off as no big deal or even justify their own tailgating behavior by saying something like: "Well, it's much worse and more dangerous to be driving slow on the freeway." This is of course utter nonsense. Tailgating is insanely dangerous, leads to a huge number of accidents, and in my mind is the equivalent of pulling a knife on someone for taking too long getting their wallet out at the grocery store.

(I would love to see widespread police crackdowns on tailgating, but I'm guessing it's just easier to prove speeding in court so that's where the tickets go. Anyone know if that's it?)

Re:Brakes. Not breaks.

[ectal (949842)]

Right. In the area I'm in, I see that kind of behavior all the time. And you're right that it's a mentality and habit that follows the driver everywhere. I've had people tailgating me through 20mph school zones. For that matter, I've had people tailgate me in parking lots.

Another interesting aspect of the problem is how many people who aren't these kind of habitual tailgaters will become tailgaters when speeds drop, e.g., on exit ramps. They maintain a safe distance until the speed gets down to 50 or 40mph. Then it's as if they forget that much past 30mph is fatal crash territory, and they plant themselves one foot off the next car.

I think the root cause for all this is a lack of respect for how potentially dangerous driving is in general. You won't find large numbers of gun owners who play with guns like toys (though they're out there), but you can find plenty of people who treat driving with all the care and attention they put into watching TV. There just isn't enough respect for driving. If there were, accident rates would plummet. I'm just going to hold out hope for a day when reckless driving is rare.

Re:Brakes. Not breaks.

[sm62704 (957197)]

Well of course you're going to need your brakes. The point is using them as little as possible. If the light ahead is red, take your foot off the gas! The idiots who live here race from red light to red light. I'm always amused when some moron zooms past me as I coast along, only to pass him as he's starting from a dead stop at the now green light.

That as well as maintaining a reasonable following distance, traffic permitting.

Your post gave me visions of Frank Drebin [wikipedia.org] hitting trash cans as he pulled up to a curb.

Re:Brakes. Not breaks.

[Midnight Thunder (17205)]

As a representative of the asshats that tailgate...try to go a little faster than the speedlimit in the left lane, you won't get tailgated as much.

In response: you can't always speed up, since sometimes the guy in front is driving slow and the people in the next lane over are driving too close to move over. You being a dangerous driver is not helping anyone and I'll more likely brake to piss you off - that way we are both being dangerous ;)

What gets me are middle lane drivers that drive at 90km/h, when the right hand lane is free, forcing everyone to over-take (ok), or under-take (not ok)

Re:Brakes. Not breaks.

[Smidge204 (605297)]

I always kinda fantasized about a switch that did the following:

  -Activate the brake lights
  -Activate a set of hydraulics to boost the read of the car up an inch or two
  -Release a little smoke from a point near the rear wheelwells
  -Play a loud screeching sound from a loudspeaker mounted under the trunk

Simulated emergency stop! Should give those tailgaters a reason to back off...
=Smidge=

Re:Brakes. Not breaks.

[Robber Baron (112304)]

I was thinking more along the lines of a depth charge style launcher that would roll a bowling ball out the trunk or tailgate when a button was pushed.

Re:Brakes. Not breaks.

[0100010001010011 (652467)]

Dude, don't loose your mind. I was just making a joke up they're. If you come off you're rocker that quickly I wonder what you have up their in your noggin. Sounds like a screw lose or something. I mean I didn't try to effect you in anyway, but now look how you've gone and disrupted the affect the original poster had. Here me out, there are a lot of people that are knew hear. You should calm down than come back later.

(stolen from myself [tdiclub.com])

Re:Brakes. Not breaks.

[The Night Watchman (170430)]

Damn you, sir! That post managed to hurt my brain even when I knew it was meant in jest. Grammar... terrible... must... correct...

This has been done before

[vivin (671928)]

Traffic Waves [eskimo.com]

That's why I never use my brakes

[Reality Master 201 (578873)]

I have respect for my fellow drivers, and only use the gas pedal. Breaking is for pussies.

Re:That's why I never use my brakes

[afidel (530433)]

Actually if you're following at the correct distance you shouldn't need your brakes in all but the most extreme situations like getting cut off. I know I try to minimize breaking most of the time and in non-gridlock situations I can keep from touching my break pedal probably 80% of the time when the car in front of me touches theirs. It requires looking several cars ahead and easing off the gas well ahead of the ripple location but if more people drove like this I bet most of those stupid sudden stop points could be eliminated.

Re:That's why I never use my brakes

[Steve525 (236741)]

Actually if you're following at the correct distance...

You, sir, have never driven on any of the highways near NY city. If you had, you would know that it is impossible to drive the correct distance behind the car in front of you. It's not merely that you'd be only person on the highway doing such a thing (annoying the cars behind you); it's that those car lengths will instantly be taken up by people cutting in front of you. You would then be forced to slow down, and the process would repeat until you find yourself driving backward.

Re:That's why I never use my brakes

[T-Bone-T (1048702)]

When traffic gets slow, I love getting behind a semi. I might not get up to speed as quickly once I'm past the problem area but I can maintain close to the same slow speed for a long time. It makes the situation much less stressful.

Re:That's why I never use my brakes

[Sanat (702)]

"Oh, and that turn signal thingy...try using is occasionally."

Here in Appalachia if a vehicle has its turn signal on then it usually indicates that the vehicle was shipped from the factory that way.

Faulty drivers

[mhifoe (681645)]

caused when one driver breaks

Maybe some more reliable drivers would have made the experiment more successful.

"caused when one driver breaks"

[Kingrames (858416)]

Well there's your problem right there.

You wouldn't have this problem if you wrote your own drivers.

physorg

[_14k4 (5085)]

It's already been done here, on Slashdot - already solved [slashdot.org] by the math guys, outlined on physorg.

But really any time I can see math at work in my day-to-day commute, is a good day to me. Also, it's fun to reach out and "touch" the asshole 200 yards behind you...

Re:

[RyuuzakiTetsuya (195424)]

Yes, and the other part of science is...

GOING INTO THE FIELD AND DOING PRACTICAL EXPERIMENTS.

By the look of it, it looks like these researchers confirmed what the mathematicians predicted would happen. Someone brakes too hard, or too early, and the rest of the flow of traffic behind them is now all fucked up.

I swear, I've seen people bitch that "oh, problem could've been busted on mythbusters if they just did the math and left it at that" and not realize that the follow through is to... DO THE EXPERIMENT.

Re:

[_14k4 (5085)]

I mentioned no such nonsense... I was reiterating that it was shown, mathematically... and my reply stated that "omg I enjoy it when math backs up real life. btw here is the physorg link." I didn't really bitch about /. reprinting a story, or "old news" and all of that.

Frankly, I think the only reason one could do this experiment is for the excuse to drive little cars in the name of science.

So, quityerbitchin' and realize that I was backing up the experiment with math.

Re:

[Lumpy (12016)]

Yup and researched and discovered more than 5 years ago. Unless there is something vastly different from Japanese drivers compared to the drivers here in the usa it's simply confirmation that the research done by grad students here is 100% correct.

Anyone that has paid attention and driven in heavy city traffic has seen this. The hill coming into detroit on I96 you can watch in the early morning a wave of breaklights coming to you from a mile away. the undulation continues for the next 30 miles and probab

Even older than that

[Scareduck (177470)]

I read a study prepared for Caltrans back in the 70's that deduced exactly the same thing. The state of traffic "science" seems to be about repeating the same insight over, and over, and over ...

dark helmet

[twoboxen (1111241)]

I knew it... I'm surrounded by *ssholes.

Keep braking, *ssholes!

stability

[backwardMechanic (959818)]

'If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.'

Is that true? If the robots had been fixed to a set driving speed (open loop), maybe. But if the robots had some sort of collision avoidance, it could still happen. It's instability in the control algorithm, no?

I love discussing traffic jams...

[explosivejared (1186049)]

... as much as the next guy, but it's been done here [slashdot.org] many times. Slow news day I guess, but nobody is surprised by this. It's pretty much common sense.

See when you put cars in the article, that immediately takes away the ability to use a car analogy. No car analogies = no lively discussion, or something like that. It's an approximation. Adding Natalie Portman or something involving Ron Paul changes the equation slightly, but car analogies are where it's at.

Wow, big news.

[Ihlosi (895663)]

So they managed to re-create a phenomenon under controlled conditions that anyone who has ever driven on a crowded highway can readily observe ? Whoop-de-doo.

Then again, I remember seeing stuff like that back at the university, where they were trying to combine traffic models with a Kalman filter to achieve better traffic jam prediction. That was, uh, over five years ago.

It's all the aggressive drivers...

[NetDanzr (619387)]

...that cut people off, forcing them to brake. What makes this even worse here in Atlanta is the fact that nobody uses blinkers to indicate they are about to cut you off. I propose a system where cars of people who cut others off are immediately stalled. That'll help the traffic flow...

robots

[MyDixieWrecked (548719)]

If they had set up an experiment with robots driving in a perfect circle, flow breakdown would not have occurred.

I, for one, welcome our new japanese robot driver overlords.

but seriously, I take this as a hint as to what is to come in the future for japan.

Speed of light trumps wave speed

[starglider29a (719559)]

I have long thought that if there was a pair of LED's in the upper left corner of the vehicle, that indicated "at/above speed limit" or "below speed limit" this would solve many problems. The problem is that, like sound in gas, the notification to slow down is given by the car in front of you only (the molecule about to bump you).

But I could see a half a mile of cars all with little green lights, I could see (at the speed of light) the wave of yellow lights approaching and ease off the gas. The wave would be absorbed by this 'viscosity'. Traffic would flow near the speed limit or average flow rate, whichever the LED's were keyed to.

And don't even get me started on those GPS nav screens. Don't show me were I am. Show me where everyone else is. Let me see the compression 2 miles ahead and I'll chill (heh heh kinetic gas pun).

"Those who forget the past", etc, etc, etc

[Ancient_Hacker (751168)]

Very silly article. IIRC traffic researchers in the mid 60's figured out the same thing, by running simulations on a 0.22 MIPS IBM/360. In FORTRAN.

Guys, there really is a benefit to hitting the library and thumbing through back issues of ld technical journals.

Modern Marvels

[Ranger (1783)]

I saw a History Channel Modern Marvels episode in highway tech and one researcher was using computer models and he determined it only takes one car to fuck things up for the rest of us. Let me repeat that it only takes one car driving slower than the rest of us to cause congestion and traffic jams on the highway.

1998 called!

[bigattichouse (527527)]

1998 called and wants Its amazing news back [eskimo.com] Except he even built animated Gifs to illustrate!

Very cool traffic sim

[nozzo (851371)]

I played with this a couple of years back:

http://vwisb7.vkw.tu-dresden.de/~treiber/MicroApplet/ [tu-dresden.de]

shame this post is buried down deep :-(

Re:no such thing as perfection

[RyuuzakiTetsuya (195424)]

You're missing the point of the experiment. Yes, in reality, something like that is incredibly likely. But the idea here is to study the effects other humans have on each other in dense driving situations.

Re:

[gwait (179005)]

Agreed, this is a classic feedback loop control problem, nothing really new at all, except that an electronic control system could easily iron out the resonances in this case.

http://en.wikipedia.org/wiki/Control_theory [wikipedia.org]

In the human case the basic problem is with reaction time, a little worse than a tenth of a second.

Say a driver slows down for 5 seconds then returns to normal speed. The one right behind him has to slow down, but takes a tenth of a second to respond. The one behind him also has to slow down,

Re:

[Goffee71 (628501)]

The M25 they mention isn't a perfect circle, which is a shame and most non-android drivers studiously ignore the speed restrictions until everything grinds to a halt. Its either 80 or nothing on the road and all the science in the world won't help.

Re:

[foobsr (693224)]

I can't find the reference, but clearly remember reading about the physics of traffic jams 20 years ago.

I do as well, and I recall there was even software (e.g. GPSS) to simulate the phenomenon. But nice to see how an experiment validates historic findings (which have probably not made it to Google yet and thus practically do not exist).

CC.

Re:Interesting, but not a solution

[FuzzyDaddy (584528)]

As someone who has been commuting at least 45 minutes for most of my career, I've had a lot of time to think about traffic. I've always thought that traffic can be compared to a phase transition, such as ice freezing. Now this is fuzzy, I haven't done mathematical models or anything.

Breaking or other external factors (an accident or flashing lights by the side of the road) can certainly precipitate a change from a swiftly moving flow to a slow moving flow. However, they only cause a transition when the density is high enough. If there's an accident during a low traffic time, you whiz by it. If they close two lanes out of four, and it's low traffic, you get a little backup, but it reaches a modest steady state size in low traffic. In high traffic you get a "wave" - the backup moves steadily backwards from the scene of an accident, and remains after the accident clears.

I often tell my wife that I can tell if a slowdown is just due to high volume or an accident by the abruptness of the slow down. An abrupt slowdown, I think, means heavy traffic "precipitated" into a jam by an external event.

So braking as described may be a precipitating event, but it's the sensitivity of the traffic flow to it that is the fundamental issue. I'd guess that even if people didn't brake so much, in those sensitive conditions a fender bender by the side of the road could cause a major backup.

(Clearly, I've thought about this WAY too much.)