Why a Traffic Flow Suddenly Turns into a Traffic Jam (2013) | Hacker News Reader

Why a Traffic Flow Suddenly Turns into a Traffic Jam (2013)

nautil.us

62 pointsdnetesn42 comments

In the US one of the biggest problems I have experienced is that drivers do not adhere to the international rule of having slow drivers on the right side of the freeway, especially in California.

As a result to this there are more lane splitting episodes (even within the speed limit) in order to finally being able to keep moving at a faster speed because the lane is effectively blocked by other drivers.

In addition to that, occasionally drivers will go faster (emergency, being late, and so on) and those same slow drivers that occupy each one of the 5 lanes at the same time will amplify the lane splitting, ultimately causing temporary traffic jams.

I never really understood why is this - in Europe we automatically merge to the right and always try to keep the leftest lane available for whoever needs to use it.

California drivers do tend to not “keep right”, partly because the highway designs that Caltrans uses tend to prefer angled-merge on-ramps rather than parallel acceleration lanes.

Drivers soon learn that the rightmost lane often has congestion due to merging cars arriving at the merge point in groups created by off-highway traffic lights, and then tend to avoid driving in the right-most lane. Metering lights on the on-ramps help reduce the grouping, but the angled merges remain, as does the learned behavior.

Also, many US states post signs: “Keep right except to pass” but California posts “Slower traffic keep right” and since nobody wants to identify as “slower” we do not.

Interstate 5 in the Central Valley is generally two lanes, and semi-trucks and vehicles pulling trailers are limited to 55 mph, and must keep right except to pass. Other traffic has a higher speed limit. When traffic is heavy, nobody leaves the left lane because they are much less likely to find a gap to merge into.

As a fellow European with experience driving in California (who prefers driving in the right-most lane), I believe it boils down to a few ways in which CA freeways are different to those in Europe:

  - posted speed limits fairly low (typically 65mph)
  - *much* higher traffic volume
  - traffic consisting of ~95% cars
  - left-lane freeway ramps
  - ramps usually merging directly into the freeway (mentioned by another comment here)

A combination of these factors results in all lanes moving at roughly the same speed (a few mph above the posted limit), which may feel bizarre to a European driver, but certainly improves traffic flow in this specific scenario.

In general, freeways are windy (similar to most European 'dual carriageways'), which requires low speed limits, which reduces throughput. Since the traffic volume is very high, to increase throughput left-lane ramps and ramps merging directly into the freeway are introduced, which may cause occasional hiccups, but on average works well. Since the vast majority of vehicles are cars, all travelling at roughly the same speed, the highest throughput can be achieved by fully occupying all lanes.

There was an interesting related discussion on HN on a few occasions about the best way to utilise escalators at metro stations - see e.g. https://news.ycombinator.com/item?id=10916704

> which requires low speed limits, which reduces throughput

Lower speed limits usually increase the density on the road which does increase the Fotoalbum throughput. It takes longer for an individual car to make a given distance, but the number of cars that pass any given point at a specific point in time is increased.

in California, at least SoCal, this "rule" has no meaning, as the congestion is at maximum level. there are simply too many cars for fast traffic to even be possible. even during non-rush hour periods the congestion is ridiculous. the only time I've driven comfortably on the highway is after 11pm and before 6am.

This used to bother me, then I realized that under heavy congestion conditions, you probably get higher overall throughput on the highway if no one tries to keep a lane free.

This is far from universal in Europe. In France, in places where highways have at least three lanes people tend to drive in the middle lane even when traffic is light. Most of the time, outside cities, merge lanes tend to be long enough even for trucks to be able to speed up enough and onramps are relatively far between so I doubt the reason people drive in the middle lane is to avoid incoming traffic.

I could see how around Paris this might be learned behaviour for a reason similar to that described in SoCal by a sibling poster. The Périphérique (the ring road around Paris proper) has mostly 3 or more lanes and looks like a highway but is not actually one. The most important difference is that traffic on the Périphérique has to yield to incoming traffic. Of course, onramps being relatively short and with bad visibility coupled with the fact that Parisians don't reliably yield when they're supposed to, has the effect of slowing traffic a lot at merge points which makes it a pain to drive in the right lane even if the traffic is relatively light.

There are few 3 lanes roads in France. It's mostly the highways that cross France east-west or north-south and a couple around the biggest cities.

On highways, the right most lane is occupied by trucks. You have to stay in the middle lane if you drive at the speed limit (130 km/h), otherwise you're swapping middle and right twice a minute to pass trucks.

The peripherique around Paris is saturated. All the lanes are used at all times. It's not possible to favor the right most lane.

Interesting fact. There are special rules in the french driving code for continuous stream of vehicles, for example it's prohibited to change lane unless taking a turn.

But it's still forbidden. In Spain it's the same but the rule you learn is "always on the right, except when you're going faster than the cars on the right". And you can't overtake a car when you're on the right, it is only allowed when you're on the left. But there's a lack of enforcement as it is also sometimes difficult to know when someone is breaking that rule.

In the UK this is illegal, since 2013 police can fine up to £100 and three points on your license however I don’t think it has made a blind bit of difference. Aggressively moving to the slow lane after overtaking sometimes helps..

Don't forget to give your windshield a good wash just as you're moving in front of that middle lane driver!

I believe its because of what some of the above commenters discussed: that some drivers believe not only do they have a "right" to the lane, but its their job to enforce the speed limit.

in Pittsburgh, part of the reason is that there are some exit ramps off the left lane. People who are afraid to change lanes get in the left lane and stay there for miles so they don't have to merge to get to the ramp

And we have weaves (2 roads merging and then splitting) that are much too short, and on ramps that have stop signs as you enter the highway. Sometimes the right becomes an exit only as well. Our roads are a mess, but I love them. Driving almost anywhere else feels easy.

One of the biggest problems I experience on freeways in the US is that traveling 5 or even 10 mph above the posted speed limit makes you a "slow driver". So even though you might feel that travelling at or above the maximum legal speed limit should give you the right to occupy the "fast" lane someone will prove you wrong by screaming up your ass at 80 mph, 90 mph, or more, flashing their high beams and becoming irate.

You don't get the "right" to "occupy" the left lane just because you're going fast. You have the obligation to yield the passing lane to anyone who wants to go faster.

You do not know who has an emergency, and who doesn't. You don't get to enforce one law (speed limits) and flout the other (yield the passing lane). In fact, you don't get to enforce any laws if that isn't your job, and if it were your job, you should not disregard one of those laws.

Hey, I exclusively drive the speed limit in the right or middle lane. This is just an observation I've made that seemed relevant. For what it's worth, I highly doubt that there was any sort of "emergency" involved in the vast majority of times I've observed this behavior. Also, apart from properly lighted emergency vehicles no one has the right to exceed the speed limit regardless of the situation.

Edit: According to this resource[0] only 4 states require you to yield to faster traffic approaching from behind in the left lane, so this seems to be far from a universal requirement.

[0] https://www.mit.edu/~jfc/right.html

What is the rule exactly?

1) Yield the passing lane for anyone that wants to go faster

2) Yield the passing lane if lane on the right is free.

I am asking because I am not sure, but (2) would make more sense to me. You should not be required to break the speed limit to be allowed to pass the slower traffic.

It seems to vary by state (see the link I posted in a sibling of your comment) but the general rule is that the left lane should only be used for passing in which case logically there should only be one "pass" happening at a time so yielding is not an issue.

Of course, in heavy traffic this goes out the window.

The rule is mostly (varies by state) that if there are two lanes, or one lane and an improved shoulder (i.e., wide enough for a vehicle), then at your earliest convenience you must yield the left lane (or the only lane) to passers.

It's very, very simple.

BTW, in Texas it is now legal to pass on the right because of all those who fail to yield the passing lane. Passing on the right isn't very good, but it's what you get when people decide that the left lane is for themselves.

This. I've had this discussion with many people who think they're entitled to use the left lane if they're going the speed limit or above, because they see themselves as enforcing the speed limit for the people behind them. I ask them to show me their badge, and when they cannot I explain that they have no authority to enforce speed limits without one. Most of them still don't get it.

You don't get the right to speed due to an emergency. Call an ambulance.

To your other point, it is not "flouting the other" law (yield the passing lane) if its 1 person doing 100 mph trying to pass you. The law is in respect to the "normal" flow of traffic, not the fact that any one person is trying to go faster than you.

When my father had a heart attack, I was told to drive to the hospital, which was a ~10 minute drive (at the speed limit). Another family member called the hospital to let them know we were coming.

During this drive, the speed limit mattered very little to me, and I drove as fast as I possibly could.

Whether this was "right" or "wrong" isn't something that crossed my mind at the time. When one's own life (or the life of a loved one) is on the line, people have a tendency to do things that don't make sense to an outside observer.

Luckily for us, we arrived at the hospital without further incident, and my father was quickly cared for. I didn't time the trip, but I imagine it was only marginally faster than driving at the speed limit, as my vehicle at the time wasn't particularly fast.

I would have done the same thing.

It doesn't mean there is a law granting us permission to do so, which was the part of the comment I replied to that I took issue with.

Its just something that is gray and we have to do when we need to.

It is the same logic as the, "I'd rather be judged by twelve than carried by 6" crowd, interestingly.

I don't care what you think of whoever is trying to pas you, so long as you let them pass. That's the law. Let law enforcement worry about who is violating it; you follow the law.

'You don't get the "right" to "occupy" the left lane just because you're going fast. You have the obligation to yield the passing lane to anyone who wants to go faster.'

Not directed at OP, but I wish more people understood this!

While I think you shouldn't be a self-righteous bottleneck on the left lane, it is clear that speeding is a major factor in these highway traffic jams. It makes it impossible to keep reasonable gaps between cars as drivers always try to go as fast as possible, getting too close to the next car in the process. This always makes me go crazy when driving in the US (although this problem is probably pretty universal to any country with a lot of traffic).

Yes, I experience this issue most commonly when there are two lanes and I am using cruise control. I come up behind a truck or a bus and I need to pass it, but someone is coming down the passing lane like it's their own personal express lane. If I change lanes and pass at the speed limit then the person behind me will catch up quickly and start high beaming me. The alternative is I disengage my cruise control and hang back, allowing the speeder to pass first, but this plan often backfires in denser traffic because speeders often run in packs and other cars will jump over and immediately accelerate, after letting them pass and merging behind them they will vary their speed erratically as they try to sort themselves back into the right lane (or not, some of them of them just keep barrelling down the left lane forever). Usually in this situation I'll try to compromise by accelerating to +10 over during the pass, but I am not really comfortable doing it and often it's still not enough to stop me from getting flashed at.

This article doesn’t mention a strategy you can take for breaking the wave:

http://trafficwaves.org/

I originally found this info via the happy mutants at boingboing:

https://boingboing.net/2013/06/23/howto-singlehandedly-erase...

After using this technique for years, I can tell you that changing your mode of driving when you encounter traffic is difficult. Especially when you’re not used to “letting people pass”. But I’m convinced it helps break the jams.

I wonder if the autonomous vehicles in our future could adopt a similar strategy when encountering traffic to reduce jams for all of us even if they’re a small percentage of the overall drivers.

I liked the article, but I agree they should have made the way to correct this more clear. They briefly mention to increase you 'headway' in the end. I wish they would have hammered this point home, instead of waxing poetically about fluid-dynamical models, & detonation waves.

The articles you linked do a great job at explaining how to fix this. Thanks for that! I believe this information should be taught as a basic part of drivers education.

I intuitively started doing this many years ago. To me, it was obvious. You can see the wave of break lights in the distance, and the wave is slowly moving back towards you. People are so impatient, they insist on riding the bumper of the car in front of them, even in a traffic jam. Then they are caught in the cycle of accelerating quickly, then coming to a full stop.

It's a simple fix. Drive slowly and steadily. Keep a large number of car lengths between the car in front of you. If you can manage to maintain the gap in front by the time you come to the back of the next 'jamiton', without breaking or stopping, the Congratulations! You just 'erased' that jam, or the wave of cars coming to a stop in that part of the jam.

All of the cars behind you will benefit. And if enough drivers were practicing this, jams would be less & clear up faster.

You are right though, the hard part is learning to not care when impatient drivers angrily speed around you, only to run up to the back of the (clearly visible) stopped cars. I just smile and laugh at how silly they are, pat myself on the back for being a smart driver, and a good citizen to all the cars behind me ;)

My view of jams is they are caused by three expectations drivers have: 1) that the speed limit is still the “minimum speed” recommendation in heavy traffic. Ie, if it’s 65, one can still attempt to drive 74. 2) that the left lane is going to be the fastest in heavy traffic and 3) that somehow using the soon-to-be-closed lane in a lane closure/traffic shift scenario is “skipping the line.” In my view if we had adaptable speed limits /expectations of speed, we could probably avoid jams altogether if everyone agreed that 35mph was an optimal speed for heavy traffic. There would be far fewer brake lights and people would have far better reaction times (provided the slower speed didn’t give them false sense of security leading to checking their cell phones). In any case, try this the next time you’re in a jam- move to the right most lane. Not only is everyone around you doing the opposite (trying to go left for fast), but there are also people exiting the highway altogether... creating a slightly faster moving lane.

> This article doesn’t mention a strategy you can take for breaking the wave: http://trafficwaves.org/

Thank you for sharing, what a pleasant read. Every time I visit my home city I'm shocked at the amount of cars in the street and consequently the traffic jam. I cannot begin to imagine how people feel after a long day at work, and what seems an even longer commute. No wonder everyone is on edge!

I think such strategies should be taught in driving schools, not only rules of the road and how to maneuver a car.

I think it was that same article that inspired me to write an iOS app that displays your running average speed and notifies you if you deviate from it.

It's been years since I've last built and run that app, but the source is here if anyone wants to take a look: https://github.com/robinmj/DeTrafficator

I've been intrigued by traffic after moving to SoCal from Oklahoma. day after day you will hit pockets of slowness only to find out there was no obstruction.

in the past I read about 'shockwaves' in traffic. one guy changes lanes too fast, which causes multiple lanes to hit their brakes, and then you get the shockwave effect ("Jamiton" in this article) and it lasts a long time. but after doing enough highway driving, I don't think that's the main issue.

my hypothesis is it's simply merging traffic. I've noticed on my daily routes that traffic is always bad where there are a bunch of roads merging, and immediately after all the merges finish, traffic speeds way up. bad drivers aren't causing these shockwaves, it's all the sudden merging. I attribute this to bad planning. "hey, this is a major road and it's going across the highway, it should probably have a merge here."

a lot of people bring up zipper merging - better training of drivers will help a bit, but properly planned highway merging would be much more efficient.

Yes. The Highway Capacity Manual is a compendium if work by the traffic engineering professionals that examines data and observations to develop theories and approaches to traffic.

If you have a freeway with no on ramps nearby (usually 6 miles) then you should be free of traffic issues if the roadway throughput (1,500 per lane per hour), if it goes above that then crazy agents in the traffic do not allow for distances between vehicles and the brake of one individual creates a cascading effect. This is one cause of back ups.

The next is if you have an on ramp on said freeway, there must be enough capacity for the freeway to absorb those vehicles in. Additionally, you need to have space and time for the vehicle to merge in. If the driver does not feel safe or have enough distance, they will perform erratic moves to get on, which will slow down traffic in the aforementioned cascade.

If a highway on-ramp is failing, it is usually due to a political person pushing for it. The Federal Highway Administration (FHWA) has a long list of requirements for a new ramp, and they are very well thought out to prevent issues and are good to work with.

I've found that a lot of times it's not an onramp merging into you, it's people waiting until the last second to merge to an offramp and slamming on the brakes and/or cutting off someone to not miss the exit.

I deal with this every day when I come home from work on my exit off the 405. People don't want to wait in line because it's slow, but it's slow because those people cut in at the last second and cause the entire lane to apply the brakes.

The 405/55 interchange is another example of that, particularly 405N to 55N in the evenings.

You are absolutely correct. In my experience, traffic is caused by drivers who 1) refuse to plan ahead and 2) are fine with fucking over other people in an effort to mitigate that lack of planning ahead.

You're being generous by saying they "refuse to plan ahead". I think they are actively planning to "cheat" and cut in line, thus screwing over all traffic behind them.

I deal with a merge every day where 3 interstates come together. The first two (I64E, I195N) run together for 1/2 mile and finally merge from 2 lanes to one just before they merge into a dedicated lane on the destination interstate (I95N). They recently (last year or so) added the dedicated merge lane on I95N to try to speed things up, but it has not helped. The real problem is people waiting until the last second to merge 2:1 for the merge on to the destination highway.

I think things would be more effective if traffic from both interstates were forced to merge to a single lane as early as possible. Then traffic would be moving at close to the speed of the destination highway, rather than at 5mph due to all the lane jockeying resulting from the 2:1 merge just before the final merge onto I95N.

What I see here in Maryland is that there is plenty of room for every car to merge onto the highway, except that nobody moves left to let them in. Why would you stay in a lane where you can see you're going to be slowed down and even have to brake, when you could move over, maintain your speed, and make room for others? I don't know, but that seems to be the majority's instinct.

Perhaps they are waiting for the next exit. Then they'd be moving out and then coming back in. Going slowly but guaranteeing your slot in to your exit is better than keeping speed and not being able to get back in.

You can't trust the other traffic to give you room. After all, you look like a guy who just decided to overtake one exit before his own.

There's been a big investment in the UK recently in "smart motorways"

http://www.itsinternational.com/categories/detection-monitor...

https://www.rac.co.uk/drive/advice/driving-advice/smart-moto...

These have overhead screens which show the speed limit (it's variable) and whether each lane can be used. They also have a lot of speed cameras enforcing that the displayed speed limit is being followed.

The intention is to keep traffic flowing at a much more consistent speed to avoid bunching. It also means if something happens traffic can be slowed down many miles away.

It essentially means the same road can have 30% greater capacity.

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