STRANGE as it sounds, adding more cars might help to clear congested roads.
Two physicists have come up with a method for optimising traffic flow that gets
traffic moving safely and quickly by sometimes increasing the number of vehicles
on the road.
快猫短视频s have likened traffic to a gas that becomes liquid and eventually
solid as congestion builds up
(In Brief, 15 November 1997, p 29). But last
month, Bernardo Huberman of the Xerox Palo Alto Research Center, California, and
Dirk Helbing of the University of Stuttgart reported an analysis of Dutch
highway traffic that found evidence of a 鈥渟ynchronised鈥� state in which vehicles
move very efficiently.
In synchronised traffic, the distance between cars is fairly short, so it鈥檚
difficult to change lanes, yet the gaps are large enough for cars to move
quickly (Nature, vol 396, p 738). 鈥淭he traffic collapses into a single
thing,鈥� explains Huberman. 鈥淚t鈥檚 moving like a solid block.鈥� Instead of having a
spectrum of cars travelling at a range of speeds, braking and swerving around
each other in an inefficient manner, the traffic moves as a solid, fast-moving
unit.
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Using these observations, Huberman and Helbing have developed formulae for
optimising the flow of traffic, based on stock market strategies. Just as a
stock trader can use formulae to get good returns while minimising risk,
Huberman and Helbing use similar equations to balance the seemingly opposing
aims of keeping the traffic moving in a synchronised manner and increasing
average vehicle speeds.
Some of the implications seem bizarre. According to their theory, it鈥檚
possible to speed up traffic not only by selectively removing cars from the
highway, but also at certain times by adding vehicles in order to maintain a
regulated flow. 鈥淚t鈥檚 a little bit wild,鈥� admits Huberman.
They have submitted their model for publication. Ultimately, they hope that
the model will be picked up and adapted by traffic regulators in different
countries.
Their formulae tell traffic controllers how to manipulate the rate at which
cars and trucks enter the highway鈥攆or example by changing traffic
lights鈥攖o ensure that traffic moves as a synchronised flow. 鈥淵ou inject
vehicles into the gaps, getting the density of vehicles so traffic is moving
like a solid block,鈥� says Huberman.
They say temporarily diverting traffic from highways is another way in which
optimum flow can be maintained when there is too much traffic.
Civil engineers currently rely on speed limits to regulate traffic. Every 500
metres along the M25 motorway that circles London, for instance, there are
detectors that determine when to trigger speed limit notices. 鈥淏y monitoring
flow and speed, you can tell when you are entering the state between free flow
and congestion, and you try to hold it in the transition phase,鈥� says Stephen
Clark, a traffic expert at the University of Leeds.
But Huberman and Helbing鈥檚 work might give engineers a new set of tools to
change the flow of traffic, though they admit it鈥檚 still too early to say how
practical the models will be.