Motorists who are too aggressive or
too timid in their driving style
are the cause of major traffic jams,
scientists have discovered.
Traffic jams are often caused by the
stop-start driving typical in
aggressive drivers Photo: ALAMY
While the underlying cause of a jam
might be an accident, a bottleneck,
or drivers simply changing lanes on
busy roads, it is how the drivers
react in the cars behind that cause
traffic to slow to a halt.
Researchers say aggressive
motorists, who drive too fast and
too close to the vehicle in front,
or timid motorists, who leave too
big a gap, send a "wave of
deceleration" backwards down the
road until traffic grinds to a stop.
Such behavior leads to the
stop-start traffic jams which
Too Many Cars Cause Traffic Jams
A Japanese team has found the
underlying cause of traffic jams
when there is no obvious reason for
Many traffic jams leave drivers
baffled as they finally reach the
end of a tail-back to find no
visible cause. An accident?
Construction work? A bottleneck? No,
just too much traffic, says a team
led by Prof Yuki Sugiyama of Nagoya
University, who has spent more than
a decade puzzling over the problem.
In the New Journal of Physics a
study by his group explains why
we're occasionally caught in jams
for no obvious reason.
The real origin of the snarl up
often has nothing to do with obvious
obstructions such as accidents or
construction work but is simply the
result of there being too many cars.
The team discovered the importance
of traffic density by applying
techniques to model the movements of
lots of particles to real-life
moving traffic. The research shows
that even tiny fluctuations in
car-road density cause a chain
reaction which can lead to a jam.
The team also studied cars driving
around a circular track with a
circumference of 230m. They put 22
cars on the road and asked the
drivers to go steadily at 30km/h
around the track. While the flow was
initially free, the effect of a
driver altering his speed
reverberated around the track and
led to brief standstills.
Prof Sugiyama says, "Although the
emerging jam in our experiment is
small, its behavior is not different
from large ones on highways. When a
large number of vehicles, beyond the
road capacity, are successively
injected into the road, the density
exceeds the critical value and the
free flow state becomes unstable."
research suggests that it might be
possible to estimate the critical
density of roads, making it possible
to build a road fit for the number
of drivers that need to use it.
Mathematicians led by Dr Gábor Orosz
of the University of Exeter have
done similar work and he comments:
"Many researchers believe that the
effect of spontaneous jam formation
(caused by tiny fluctuations above a
critical traffic density) is the
main reason for traffic jams and
this view is supported by Prof
The Exeter work is different because
the reaction time delay of drivers
is included, revealing that the late
reaction of drivers even one second
can have big knock on effects when
driving at much higher speeds than
in the Japanese study.
"In a typical situation a vehicle
dropping its speed from 128Km/h to
104Km/h may cause a ripple that
later vanishes while dropping its
speed from 128 km/h to 100Km/h may
cause a ripple that is amplified and
leads to traffic jams."
Heavy traffic on highways does not
automatically lead to congestion but
can be smooth-flowing, he says. "We
are currently developing algorithms
for radar-guided computer-controlled
cruise-control devices that could
cut down over-braking and keep