480,000
years
estimated total time
wasted in traffic by
u.S. drivers in 2007
$87.2
billion
estimated cost of
traffic congestion in
u.S. in 2007
3-d X-rays show
nanosized details
high-resolution technology
exposes a material’s insides
By Gwyneth dickey
A new X-ray microscope technique gives
a super-detailed look inside small samples, and does it in 3-D.
An international team of scientists
used the method to examine a tiny piece
of mouse bone and reveal inner details as
small as 100 nanometers, such as hidey-
holes for cells in the bone matrix and con-
necting channels between those pockets.
A fragment of mouse bone visualized by
a new technique that creates 3-d X-rays
of features as small as 100 nanometers.
in a 180-degree view of the material, a
computer program puts all the images
together into a 3-D image, similar to the
way a CT brain scan is created.
The Tao of traffic: Go with the flow
if stoplights bend to cars, a study says, the way grows clearer
By Rachel Ehrenberg
Traffic lights that act locally can cut
travel time globally, new research suggests. By minimizing congestion, the
approach could save money, reduce
emissions and perhaps even quash the
road rage of frustrated drivers.
The new approach makes traffic lights
go with the flow, rather than enslaving
drivers to the tyranny of timed signals. By tracking vehicle flow
through each intersection as
it occurs and coordinating
lights with only their nearest neighbors, a systemwide
smoothness emerges, scientists report in a Santa Fe
Institute working paper
posted online in September.
Traffic lights usually are
controlled from the top down,
operating on an “optimal” cycle that
maximizes the flow of traffic expected
for particular times of day, such as rush
hour. But even for a typical time on a
typical day, there’s so much variability
in the number of cars at each light and
the direction each car takes leaving
an intersection that roads can fill up.
Combine this condition with overzeal-
ous drivers, and intersections easily
become gridlocked. Equally frustrating
is the opposite extreme, where a driver
sits at a red light for minutes even though
there’s no car in sight to take advantage
of the intersecting green.
“It is actually not optimal control,
because that average situation never
occurs,” says complex-systems scien-
tist Dirk Helbing of the Swiss
Federal Institute of Technol-
ogy in Zurich, a coauthor of
the new study. He and Stefan
Lämmer from the Dresden
Technical University in Ger-
many decided to scrap the
top-down approach and start
at the bottom. They noted that
when crowds of people are try-
ing to move through a narrow
space, such as through a door connecting
t wo hallways, there’s a natural oscillation:
A mass of people from one side will move
through while the other people wait, then
suddenly the flow switches direction.
“It looks like maybe there’s a traffic
light, but there’s not. It’s actually the
“The
traffic flow
controls the
traffic light
rather than
the other
way around.”
buildup of pressure on the side where
people have to wait that eventually
turns the flow direction,” says Helbing.
“We thought we could maybe apply the
same principle to intersections; that is,
the traffic flow controls the traffic light
rather than the other way around.”
The arrangement puts two sensors at
each intersection: One measures incom-
ing flow and one measures outgoing flow.
Lights are coordinated with every neigh-
boring light, such that one light alerts the
next, “Hey, heavy load coming through.”
“The approach is adaptive and the
system can react,” says mechanical
engineer Gábor Orosz of the University
of Michigan in Ann Arbor. “That’s how
it should be.”
The researchers simulated the
approach for part of downtown Dres-
den. The flexible self-control approach
reduced time stuck waiting in traffic by
56 percent for trams and buses, 9 per-
cent for cars and trucks and 36 percent
for pedestrians crossing intersections.
Dresden is now close to implementing
the new system, says Helbing, and Zur-
ich is also considering the approach.
“In general these algorithms improve
traffic, but maybe not as much as they do
on paper because we are still human,”
Orosz says. “It is still humans driving
the cars.”
m. dierolf, p. thibault and f. pfeiffer/technical univ. of munich
www.sciencenews.org
