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science hobbyist traffic waves physics for bored commuters amasci com a cure zipper links faq com ments traffic waves sometimes one driver can vastly improve traffic jan 1998 william j beaty electrical engineer wall street journal main article 2016 note wait for animation to load new 2008 video one driver unclogs a seattle jam i live in seattle and my two daily commutes last about 45 minutes that s when i m lucky sometimes it s more like two hours each this has given me an immense amount of time for watching the interesting patterns in the cars boredom led me to fantasize about the traffic being like a flowing liquid with cars acting as giant water molecules over many months i slowly realized that this was not just a fantasy why had i never noticed all the traffic fluid dynamics out there once my brain became sensitized to it i started seeing quite a variety of interesting things occurring eventually i started using my car to poke at the flowing traffic observation eventually leads to experimentation no there are amazing things you can do as an amateur traffic dynamicist you can drive like an anti rubbernecker and erase slowdowns created by other drivers but first some basic phenomena invisible accidents have you ever been driving on an interstate highway when traffic suddenly slows to a crawl you inch along for many minutes while waiting to see the accident which must have caused the jam at the same time you also curse the rubberneckers who are causing the whole problem but then all the cars ahead of you take off at high speed the jam is over but no accident no police cars nothing what the heck was that a traffic jam with no cause in the rear view mirror you see all the poor saps behind you still stuck in the jam but why if all those people could just speed up at the same time the whole traffic jam would evaporate why don t they ever do that what caused the mysterious slowdown in the first place after experiencing many of these invisible accidents i came up with the following explanation to best understand this imagine that you look down on traffic from an aerial view point pretend you re in a traffic reporter s helicopter looking downwards figure 1 cars lining up behind an accident above in fig 1 i ve drawn a one lane road an accident and a row of cars stuck behind the wreck other cars are approaching from the left and stopping too suppose that the wrecked car the red one above has simply become temporarily stuck maybe it spun out on ice what will happen when the red car moves and unplugs the flow figure 2 a wave of condensed traffic creeps backwards refer to fig 2 above in the top row at 2a the flow is suddenly unplugged but not all the cars can move since most cars are stuck behind drivers who are stopped figure 2b shows the traffic a few moments later and figure 2c shows it a few moments after that notice the orange car in 2a and see how it eventually becomes unjammed in 2d and begins moving at the same time the red car in 2a approaches the jam and is swallowed up a moving wave of jam after the wreck is removed there seems to be no reason for the traffic jam to persist yet it does the reason for this is sensible if i am stuck behind a car that is stopped then i have to stop too and so does the car behind me all the cars in the jam are in this situation even though the wreck is gone they remain locked at standstill because if they want to move they all have to move at once they never do because each driver is waiting for the car ahead to move if i am in the traffic jam i m not going to move forward because i have no room to do so i d bump the car ahead of me we all think like this so none of us can move when the car in front of me leaves i still cannot accelerate instantly so i will remain stopped for a moment i must delay leaving for a moment if i started up instantly i d stay too close to the car ahead of me and that would not be safe each departing car must delay in the same way and this causes the jam to evaporate starting from the forward downstream end it evaporates in a wave which begins at the forward end of the jam near the wreck the wave eats into the jam from right to left yet new cars are piling onto the back end of the jam starting at figure 2a the cars depart from the jam in sequence in 2b the wave of evaporation has moved away from the wreck site and in 2c and 2d it is far from the wreck but notice an interesting thing even though the cars themselves are moving from left to right the wave of evaporation moves in the opposite direction it moves leftwards as it eats into the traffic jam there is a second important thing to notice while some cars are still jammed more cars are piling up behind them at the trailing end of the jam even after the wreck is removed more cars are still condensing onto the back of the jam the traffic jam is like a solid object whose front end is evaporating and whose back end is growing like a crystal cars move left to right yet look at the the group of stopped cars the stoppage is creeping slowly upstream in the opposite direction to the moving cars the accident is gone but a moving wave of stopped cars remains behind it s not a traffic jam it s a shock wave which propagates through the automotive material it s a traffic clot in the blood vessel it s a traveling wave of traffic condensation not caused by accidents these sorts of travelling waves are common during heavy traffic conditions an accident isn t needed to create them sometimes they are caused by near misses by people cutting each other off by merging lanes at a construction site or simply by extra cars entering from an on ramp in traffic engineering lingo they can be caused by incidents on the highway a single rubbernecker could cause one by momentarily stopping to look at something interesting whenever you slow way down in order to merge across a lane to get to your upcoming exit you could create one sometimes the traffic waves have have no real cause at all they arise from nothing because tiny random motions can trigger large results they are like sand ripples and sand dunes and they just build up for no clear reason they are like ocean waves caused by the steady breeze or like the waves which move along a flapping flag they just emerge spontaneously from the writheing lines of traffic in the science of nonlinear dynamics this is called an emergent phenomenon how long will the traffic wave last after the accident is cleared its lifetime depends upon the amount of traffic and on the number of cars trapped in the jam but sometimes these things can persist for many hours when traffic is slight the jam might shrink rapidly to nothing but if traffic remains heavy then there s no reason for the travelling wave to ever dissipate at all also if the conditions are just right if the condensation happens faster than the evaporation then even a tiny wave could grow large and larger sort of like dropping a tiny seed crystal into a supersaturated solution when traffic is heavy and unstable the normal faster slower noise produced by driver can make the traffic freeze into a gigantic crystal like kurt vonnegut s end of the world story cat s cradle it s the ice nine of the highways so next time you are commuting and you approach a stoppage don t think of it as a stupid f traffic jam think of it as a pressure wave which has approached your car and engulfed it think of it as a simple living thing which is composed of cars rather than molecules stay hopeful that the crystalline amoeba poops your car out soon take an aerial viewpoint and visualize the wave which is moving backwards as you move forwards next page a timeline of cutting edge traffic research 2020 new niche field is now called jad jam absorption driving 2017 single self driving car wipes out waves pdf 2013 theory of jam absorption driving nishi et al 2013 suppressing instabilities w acc 2009 traffic soliton wave model jamitons 2008 waves spontaneously grow on a real highway 2004 new discovery cruise control can erase traffic jams 2004 1st accurate sim of traffic waves some cars must aggressively accel then brake hard 2000 just one fluctuating vehicle is seed for jams 1998 traffic waves only appear during self organized sychronized flow 1998 physicists investigating traffic waves they call them the pinch effect 1998 self organized synchronous flow discovered in heavy traffic 1993 single lane simulation produces self organizing emergent patterns traffic jams 1965 fortran car following simulation cdc mainframe tractorfeed print lots more reading at univ eth zurich popular press and research papers also see my frequently asked questions and visitor comments next page 2 traffic experiments and a possible cure for certain types of traffic congestion also see best stuff here amasci com note that traffic physicists have a different name for traffic waves the pinch effect waves a cure zipper email links faq comments animated with a great little program 25 try their free eval version fyi see idleworm how to animate this is science hobbyist website main site pages science hobbyist cool science amateur science weird science close minded science teacher resources the good stuff experiments and demonstrations site nav help next page 2 traffic experiments and a possible cure for certain types of traffic congestion also see best stuff here amasci com http trafficwaves org created and maintained by bill beaty mail me at electrica technology inc and linkedin hosted by try this isp eskimo com for inexpensive access to unix shell perl scripts secure commerce server irc muds and all newsgroups fyi see the history behind the name eskimo com view my stats