Have you ever driven out of a traffic jam only to find that
nothing
was apparently causing it?
Explanation:
When the cause of a freeway traffic jam, e.g., an accident, a brief stall,
a temporarily overcrowded freeway entrance or exit, etc., is removed or is
no longer in effect, the jam does not disappear instantaneously. It takes
time to do so. This is no different from a queue of cars at a red traffic
signal, which does not dissipate immediately when the light turns green.
In both cases, freeways and streets, jams need time to dissipate, and the
heavier traffic is the longer the process. During the rush hour a freeway
jam can take much longer to dissipate than to grow in the first place.
The evolving jam moves over the freeway in the direction opposite to
traffic, affecting it far from the location of the original cause.
Drivers exiting the jam see nothing unusual, much to their surprise.
A computer animation showing the effects of a jam created by an incident downstream of a freeway off-ramp (a
"diverge") can be seen by clicking on the icon below. The "movie" is a "cell-transmission" simulation of a symmetric
freeway diverge with constant input flow where 50% of the vehicles go in each direction. Numbers in each cell are
proportional to the number of cars in the cell at the given time. Decimals have been truncated (e.g., 3.5 appears as
3). Low numbers (green) correspond to freely flowing conditions. Numbers greater than 8 (in red: 9, A, B, C...)
correspond to queues. The higher the letter the higher the congestion
The simulation depicts what happens to this system when a brief incident ( from time "70" to time "115") partially
blocks the upper branch of the diverge. Note how the queue grows, and that when it reaches the diverge shortly after
time "100" it affects traffic on the bottom branch. After the incident is removed (at time "115") the front of the
queue begins to move backwards, chasing the back of the queue. Eventually, the entire queue moves entirely upstream of
the diverge. If a driver were to pass through the queue during this phase, e.g. at time "135", (s)he would see
nothing; except that traffic would be clearing for no apparent reason. But there was a reason; it just happened
earlier, somewhere else. Note finally, how the queue eventually dissipates at time "196". The incident lasted 45 time
units but was felt for 126 units.
In reality, the fronts and backs of queues move about 4 times more slowly than shown in this animation but the effects
you have seen are qualitatively correct: real queues move less in space but the duration of the effect is similar. If
the incident happens when traffic is saturated (numbers in the cells equal to 8 in our example), or if it happens
inside a queue (numbers greater than 8), the fronts and backs of queue generated by the incident move at the same
speed and the queue does not die down. This is why during a rush hour, the effects of a 5 minute incident can often
be felt for the duration of the rush.
