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What Causes Traffic Jams!?
I, like you, have wondered for many years why we all just suddenly stop in the middle of an interstate highway without any triggering event like an accident up ahead. So I did a fairly thorough search and came up with an explanation that seems to identify all the main factors, all the main reasons for the formation of traffic jams. However, I sandwiched the one I like between two that are more typically found when searching this subject. I recommend reading as much or as little of the first and third article or excerpt as you like, but read the Freakonomics article, followed by Aaron S.'s comment. It is that comment that so succinctly identifies all the primary culprits and causes of traffic jams. He even offers solutions after identifying the causes.
You could try to read something like this mathematical explanation of traffic flow and of the dynamical phenomenon of the jam:
Mathematical models of traffic flow are constructed focusing on this simple property of the motion of vehicles. In short, traffic flow is a non-equilibrium physical system consisting of moving particles with asymmetric interaction of exclusive effect. The models have basically two kinds of solutions: a free flow solution and jam flow solutions. In a free flow solution, all vehicles move at nearly the same large velocity with a safe distance between two successive vehicles. In contrast, a jam flow solution shows traveling clusters in a flow. Vehicles almost stop in a cluster, which is identified with a traffic jam. Such models have succeeded in forming a jam on a circuit in the absence of a bottleneck, when a free flow is initially set and the average vehicle density is a little bit larger than a certain critical value, which corresponds to the critical density of fundamental diagrams.
The physical mechanism of forming a jam is summarized as follows. Small fluctuations always exist in the movement of vehicles in a traffic flow. If the vehicle density is low, such a fluctuation disappears and the free flow is maintained. On the other hand, if the density exceeds the critical value, the fluctuation cannot disappear but instead grows steadily and eventually breaks the free flow. After relaxation, a jam is created and travels along the circuit without decaying. Mathematically, the free flow solution is unstable and the jam flow solutions become stable. The stability change at the critical density caused by the enhancement of fluctuations in a free flow is understood as a phase transition of non-equilibrium systems. The phenomenon originates from the dynamical effect of the collective motion in a many-body system. It does not require the existence of a bottleneck.
Traffic flow can be investigated as a dynamical phenomenon of a many-particle system. In general, such a system drastically changes its macroscopic aspect owing to the effect of the collective motion of interacting particles. Such phenomena are observed in several fields in physics. The characteristic features of collective phenomena are phase transition, bifurcation of a dynamical system, pattern formation, etc. It is not unexpected that the same physical mechanism appears in socio-dynamical objects. Traffic jams are just the subject for investigation along this line. We have succeeded in simulating a traffic jam by mathematical models without a bottleneck, whenever the average vehicle density on the road exceeds a certain critical value. The crucial point is the effect of collective motion caused by the interaction among vehicles, which is originated by drivers seeing other vehicles. The effect makes the free flow unstable and generates a traffic jam similar to phase transitions and pattern formation in non-equilibrium many-particle systems.
Can our mathematical prediction be tested? In this paper, we report the first experimental verification that a jam can be generated in the absence of a bottleneck. Our experimental result is consistent with our simulations, and provides clear evidence that the emergence of a jam is a collective phenomenon.
In real data, the relationship between vehicle density and flow rate (fundamental diagram) has a universal property in highway traffic. The data points of traffic flow are sharply divided into two parts at the critical value of the vehicle density, labelled as free flow and congested traffic flow. In the congested flow part, it is easily supposed that a traffic jam appears just beyond the critical density. Fundamental diagrams show similar shapes at any point on any highway, and the critical density is almost the same value. Such common properties indicate that the phenomenon of traffic jams can be studied from the physical point of view as a dynamical phase transition.
Or you could read this one (much more clear and easy to understand):
Freakonomics: The Hidden Side of Everything
What Causes Traffic Jams? ...You!
The next time a traffic jam materializes in front of you for no apparent reason, think about Japan. That’s where scientists have, for the first time, recreated “shockwave” traffic jams, in which one driver’s slowing down creates a ripple effect that moves backwards through traffic, grinding everything to a halt for miles. They say recreating the phenomenon successfully is the key to finding ways to defeat it.
Their experiment found that human error is a major cause of these most frustrating kinds of traffic jams (there are, of course, other causes). But if driver error is the source of the problem, don’t drivers also have the solution? Clive Thompson points to one idea, the classic “slow down and keep a constant speed” method, which seems to be effective in breaking these shockwaves. Any other solutions?
I used to be quite amused when scientists would run traffic tests and just could not figure out why there were traffic jams! Everyone who drove could!
I even figured out how to figure it all out. Create a computer program with a highways, roads, exits, on-ramps, redlights, and the such like.
Then, ALL YOU HAVE TO DO, is make a bunch of “cars” with different parameters. ABC Cars take off like a rabbit at a green light…DEF cars take off a bit slower…and XYZ cars take off like molasses.
Then, on the interstate, you have people who exceed the speed limit, go the speed limit, and then those people who ought to be sent to prison.
Further, you permit some of the slow pokes to get in the left lane every now and then.
Also, people have different braking prowess…this means there will be wreaks at times…which causes rubbernecking until its cleaned up.
Then there’s my personal pet peeve: People who don’t know how to merge. The traffic is doing 70…and they get on doing 45…slowing everyone down.
How to fix it?
Imprison anyone who does not know how to merge correctly (or even death penalty if they’re over 40, which means they’ve been doing it for years!).
I would also add that if an accident could be gotten COMPLETELY out of view (including all the flashing lights, etc.), traffic wouldn’t slow down! So, if you have a fender bender, don’t stop in the lane…get off to the side…miles to the side, if possible.
Lastly, police ticket drivers in the left lane going under the speed limit (assuming normal conditions). This alone would reduce the national blood pressure by at least 20%.
In this instance, I am all for profiling, whether it be racial, gender, age, or whatever. If we find that a particular group of people are the worst offenders, they must be send to the penitentiary and given “Driver Sensitivity Training” until they actually know how to drive.
You might also be able to interpret relevant excerpts from the..."Emergent Phenomenon in Congested Traffic Flow"
(Daniel Vandervelde's final physics term paper submitted on 5-6-04) --Good Luck. :o)
size k is then associated with a site of the CM occupied by k particles. One then proceeds
by examining the evolution of the domains, and identifying their dynamical processes. As will be demonstrated, in many cases these processes are closely related to the diffusion and the chipping processes of the asymmetric CM.” The density of the automobiles tells the story of the movement of the vehicles. First they consider what they call the “cruise control limit”. This is where the probability of braking occurring is zero, in other words all cars are maintaining a constant velocity. There, as long as the density stays below some density max, free flowing traffic persists. Here all cars are moving deterministically, and one can express the current as J( )= max v . As density increases, local jams form and current reduces. This leads to the conclusion that a phase transition, if one exists, must occur at some 0
less than or equal to f .
This study also had some revealing results regarding the flow of vehicles out from
a jam. It was found that the outflow of traffic from a jam will self organize, creating a
critical state of maximum throughput. This state was achieved when the emergent traffic
jams were just able to survive indefinitely. “This implies that the intrinsic flow rate for
vehicles leaving a jam equals maximum throughput.” Results of this study show that
maximum throughput is actually achieved when the left boundary condition is that of an
infinitely large jam, and the right boundary condition is left open. This is explained by,
“An intuitive explanation is that maximum throughput cannot be any higher than the
intrinsic flow rate out of a jam. Otherwise the flow rate into a jam would be higher than
the flow rate out, and the jam would be stable in the long time limit, thus reducing the
overall current. By definition, of course, the maximum throughput cannot be lower than
this intrinsic flow rate.” It is true that the maximum throughput selection is something which is intrinsic in driven diffusive systems. This model differs though in that the left boundary condition is that of the front of the infinite jam drifting backward in time. “If the left boundary is fixed in space and vehicles are inserted at velocities less than max v , then the outflow from a jam cannot reach maximum throughput”. This is
particular notable since real world situations where one has a disturbance which cannot
move, like onramps or reductions in lanes, lead to lower throughput downstream than the theory would predict.
Has anyone found a simpler explanation for traffic jams on open highways having no bottleneck and no traffic accident? If so, please share it with me. Please!