Accordian Effect – Traffic Flow

by Richard Russell, columnist, GlobeAuto, The Globe and Mail


Believe it or not, there is a serious correlation between traffic flow and molecular flow in a gas. Scientists know that any slight change in the speed of a single molecule can cause a compression wave to propagate backwards. In a similar vein, the actions of a single driver can cause a compression wave behind him causing following vehicles to slow or brake.

In a perfect world, traffic could flow smoothly at high speeds with very little distance between vehicles. Stock cars race around Daytona at 325 km/hr inches apart with cars pulling into spaces less than a metre longer than the car. If we could do this on the open road, we’d get three or four times more vehicles into the same space and everyone could drive much faster.

BUT, the average driver is just that – average. And for everyone who is as gifted as a race driver, there is someone else who is below average. It only takes one of these less-attentive or less skilled drivers to screw up an entire section of highway. While it is theoretically possible for everyone to drive two car lengths apart at 120 km/hr on the 401, actually it is a common scene – four lanes jammed with traffic flowing at 120. But, it only takes one vehicle changing speed to mess up that perfect scenario.

And herein lies the rub. A single driver changing speed can interrupt flow for hours as the accordion effect takes place behind them. It is quite acceptable if that change in speed is caused by a critical issue but more commonly because that driver has had a lapse in attention or wants to stare at something. Answering or dialing a cell phone is the most obvious and frequent problem, but it could just as easily be any number of other distractions. We’ve all seen how traffic flow grinds to a near halt near the scene of a crash – even when it is in the opposite side of the road and of no consequence. But anyone who regularly uses cruise control will be all too aware of how the majority of drivers fail to maintain a steady pace, even when there are no outside influences. Watch as driver as you approach from behind and you’ll notice their speed drop if they reach for the stereo or are involved in a conversation. As you reach them and their attention goes back on the job at hand they speed up.

Another scenario that creates an accordion effect occurs when vehicles change lanes erratically – those who accelerate and brake, whipping traffic passing on both sides and between lanes in order to arrive at their destination seconds sooner. In the meantime, like a boat on a still lake, they leave behind a wake that spreads from shore to shore or across all lanes. It is ironic that you often catch up to this gifted motorist chatting away on the phone traveling 20 km under the flow. But watch out. When he hangs up and gets back to the task of actually driving, he’s going to be a force to contend with.

Traffic engineers know that as much as 75% of all traffic jams have no visible cause – that a single or few motorists slowing, can cause a compression wave to propagate rearward. The result can last for hours after the initial event on crowded roads. Drivers coming upon the scene have no clue as to the cause for the sudden stoppage. Even the slightest change in speed or lane change causing following vehicles to slow or brake can have consequences out of proportion to the event.

If we let computers monitor and maintain our speed, we could avoid most traffic jams. Scientists have actually suggested that if every vehicle on the road were equipped with adaptive cruise control – the new type that uses a radar to maintain a set distance from the vehicle ahead, traffic flow could be sped up and density increased. The problem is that many drivers would either fail to activate it or shut it off so they could stare at something alongside the road.