Wednesday, June 17, 2009

About those 'Phantom' traffic jams reports that MIT is following up on Japanese research into traffic jams that happen without having a wreck or other restriction on the road to explain why cars are stopped, waiting - for the cars ahead to move out.
A team of MIT mathematicians has developed a model that describes how and under what conditions such jams form, which could help road designers minimize the odds of their formation. The researchers reported their findings May 26 in the online edition of Physical Review E.

Key to the new study is the realization that the mathematics of such jams, which the researchers call "jamitons," are strikingly similar to the equations that describe detonation waves produced by explosions, says Aslan Kasimov, lecturer in MIT's Department of Mathematics. That discovery enabled the team to solve traffic jam equations that were first theorized in the 1950s.

So I wrote to Elizabeth Thomson at MIT.
Ms. Thomson,

Some time back I listened to the audio book, "The Goal", Eliyahu M. Goldratt, circa 1993-94. The premise of this business book was to apply physics understanding of interdependent events to improve productivity.

Application to traffic seemed obvious. Interdependent events, cars following each other in lane of traffic, combined with statistical variation - the different acceleration and braking rates of vehicles, determined by vehicle loading and capacity, and driver inclination, are the reason for traffic jams.

Given a vehicle encroaching on the vehicle behind - by driving slower, entering a busy lane too close to the following vehicle, or braking for a real or perceived hazard, causes all vehicles behind to slow. Unfortunately, the various rates of speed recovery of the affected vehicles prevents complete recovery of the preceding progress of vehicles down the lane of traffic.

The solution seem obvious to me - enforce existing laws about safe following distances. Enforce laws preventing a vehicle changing lanes to close to a vehicle already in the lane ('cutting off' the following vehicle).

Safe following distances are set by *posted* speed limits, not vehicle speed. Enforce safe following distances even when stopped for the vehicle ahead - and traffic jams will evaporate on their own. You also lose the critical aggregate density of idling vehicles that contributes to dense air pollution due to traffic jams.

I recall my driving-ed instructor explaining about pulling up behind the vehicle ahead when stopping at a stop light. I believe this "common sense" rule actually violates traffic laws regarding safe following distance - and compromises the ability of roads to handle heavy traffic rates. In all instances, I think keeping the prescribed safe following distance from the vehicle ahead *accelerates* traffic throughput as a function of vehicles per unit of time.

When a driver experiences dismay or fear at road conditions, and expresses distress by slowing, I consider that a "flinch". Flinching occurs when someone changes lanes or enters the roadway too close to oncoming traffic, and any oncoming driver slows whether to avoid an accident, or because they, rightly or wrongly, fear the potential of a collision. The flinch affects cars behind, because they then flinch - and pull closer to the vehicle ahead as the vehicle speeds decline. This compression of vehicle spacing in distance is directly related to a compression of vehicle spacing in time - and each vehicle will recover some or all of the previous speed, depending on space to the vehicle ahead, speed of preceding vehicles, etc.

The region where traffic compression in time occurs can be thought of as a source of vehicles. Until all vehicles involved have been released from the compression, that is, the traffic jam works it's way through, the likelihood is that additional vehicles will continue to arrive at the back of the queue and stacked up for later release, before all preceding vehicles compressed in time and distance are released at their lower speed and higher density. The higher density, or compressed time, restricts the ability of any individual vehicle to resume free-road speed and separation in time and distance.

My solution is to ticket everyone in a traffic jam that comes to rest or approaches the vehicle ahead within the safe following distance. This brings awareness of this particular law to the attention of drivers. Avoiding tickets by coming to rest safely spaced, drivers reduce the constraint on easing the traffic jam by relaxing the compression restraint that slows relief of the jamiton.

This is a community-type suggestions. A driver does nothing for himself/herself by maintaining a safe following distance (except for reduction in driving stress, reduction in risk of collision, and avoiding violation of the laws about safe following distances). Traffic wise, the driver that maintains the safe following distance only reduces the likelihood that some drivers *behind* the vehicle will be involved in a slow-down. Keeping the safe distance back reduces the likelihood of a flinch, and the reduced compression factor permits release of the jam before it forms - before another vehicle can arrive in the region and experience the cascading compression we call a traffic jam.

Brad Kruse
Ponca City, OK

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