7 ms·

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

62 points by dnetesn 8y ago

fosk 8y ago
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.
- theonemind 8y ago
  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.
- was8309 8y ago
  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
  - jimktrains2 8y ago
    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.
- dmckeon 8y ago
  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.
  - deleted 8y ago
    [deleted]
- legohead 8y ago
  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.
- c22 8y ago
  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.
  - cryptonector 8y ago
    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.
    - c22 8y ago
      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 https://www.mit.edu/~jfc/right.html
    - yeuuehhr 8y ago
      Fuck you and follow the law dip shit. You're going to get people killed because you feel entited to drive like a maniac.
    - Scea91 8y ago
      What is the rule exactly? 1) Yield the passing lane for anyone that wants to go faster or 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.
- deleted 8y ago
  [deleted]
- vladvasiliu 8y ago
  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.
  - aarroyoc 8y ago
    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.
  - user5994461 8y ago
    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.
- LiamPa 8y ago
  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..
  - McDev 8y ago
    Don't forget to give your windshield a good wash just as you're moving in front of that middle lane driver!
- alor 8y ago
  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 https://news.ycombinator.com/item?id=10916704
  - Xylakant 8y ago
    > 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.
- apple4ever 8y ago
  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.
deleted 8y ago
[deleted]
legohead 8y ago
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.
- Agustus 8y ago
  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.
- robmaister 8y ago
  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.
eljefe900 8y ago
This article doesn’t mention a strategy you can take for breaking the wave: http://trafficwaves.org/ http://trafficwaves.org/ I originally found this info via the happy mutants at boingboing: https://boingboing.net/2013/06/23/howto-singlehandedly-erase-tra.html 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.
- 40four 8y ago
  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 ;)
  - mattrp 8y ago
leoedin 8y ago
There's been a big investment in the UK recently in "smart motorways" http://www.itsinternational.com/categories/detection-monitoring-machine-vision/features/uk-defaults-to-hard-shoulder-running-to-expand-motorway-capacity/ http://www.itsinternational.com/categories/detection-monitor... https://www.rac.co.uk/drive/advice/driving-advice/smart-motorways/ 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.

62 points by dnetesn 8y ago

fosk 8y ago

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.

theonemind 8y ago
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.
was8309 8y ago
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
- jimktrains2 8y ago
  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.
dmckeon 8y ago
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.
- deleted 8y ago
  [deleted]
legohead 8y ago
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.
c22 8y ago
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.
- cryptonector 8y ago
  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.
  - c22 8y ago
    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 https://www.mit.edu/~jfc/right.html
  - yeuuehhr 8y ago
    Fuck you and follow the law dip shit. You're going to get people killed because you feel entited to drive like a maniac.
  - Scea91 8y ago
    What is the rule exactly? 1) Yield the passing lane for anyone that wants to go faster or 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.
deleted 8y ago
[deleted]
vladvasiliu 8y ago
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.
- aarroyoc 8y ago
  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.
- user5994461 8y ago
  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.
LiamPa 8y ago
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..
- McDev 8y ago
  Don't forget to give your windshield a good wash just as you're moving in front of that middle lane driver!
alor 8y ago
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 https://news.ycombinator.com/item?id=10916704
- Xylakant 8y ago
  > 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.
apple4ever 8y ago
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.

deleted 8y ago

[deleted]

legohead 8y ago

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.

Agustus 8y ago
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.
robmaister 8y ago
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.

eljefe900 8y ago

This article doesn’t mention a strategy you can take for breaking the wave: http://trafficwaves.org/ http://trafficwaves.org/ I originally found this info via the happy mutants at boingboing: https://boingboing.net/2013/06/23/howto-singlehandedly-erase-tra.html 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.

40four 8y ago
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 ;)
- mattrp 8y ago

leoedin 8y ago

There's been a big investment in the UK recently in "smart motorways" http://www.itsinternational.com/categories/detection-monitoring-machine-vision/features/uk-defaults-to-hard-shoulder-running-to-expand-motorway-capacity/ http://www.itsinternational.com/categories/detection-monitor... https://www.rac.co.uk/drive/advice/driving-advice/smart-motorways/ 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.