29 June 2013

Your Actions are (part of) Causing that Traffic Jam You're Stuck in*

*In the morning and evening of most large American cities (especially those surrounded by plenty of suburb), when everyone is driving their cars to their 9-5 jobs, there are simply too many vehicles on the highway for the lane capacity.  You get on the highway at the nearest entrance, and proceed to average 15mph the entire distance from your suburban home to the downtown city center where you work, frequently coming to a complete stop, never going more than 25mph at the most.

In that situation, traffic is going to go slow, no matter what.
That isn't the type of traffic jam I'm talking about.
There is also another type of traffic back up.  The kind that happens in moderate traffic.  Everyone slows down, sometimes even to a complete stop, and then a few hundred feet later, you are moving again at 50, 60, 70mph, as if nothing happened.
Sometimes this happens because there is the aftermath of a crash in the shoulder, or even across the divider on the opposite shoulder of the oncoming lane, and all the drivers feel it is very important for them to take a good look at it, because humans are just like that.  Other times its because someone is getting a traffic ticket, and, even though the cop is clearly busy at the moment, people imagine they are more likely to be caught speeding if the can see a police car.
But most often, these slow downs happen for no apparent reason at all.  You get to the front of it, and cars are accelerating just as suddenly as they slowed down.
Sometimes traffic pulses like this, fast - slow - fast - slow - fast - slow for miles.  In some places, not quite as dense as in the first example above, the daily commute does this pulse jam every single day.
When you find yourself in this situation, the choices you make can either make it better, or they can make it worse.  If you are reading this, there is a decent chance you are one of the few who makes it better already - but if you are like most people, there is a much better chance you are making it worse. 
In fact, if everyone realized what I'm about to explain, and acted appropriately, those slowdowns would never happen in the first place, but, of course, most people don't know any better, so its hard to hold it against them.
At least once you have finished reading this, there will be one more person who understands whats going on, and makes it better instead of worse.

The easiest way to understand how individual actions make the backup better or worse is with an analogy.

Lets say you are in a crowd, and for some reason everyone wants to go through a doorway as quickly as possible (the iconic burning theater, perhaps, or maybe just a Black Friday sale).
Each individual is acting as an independent free agent, and each wants their own personal speed to be as fast as possible.
What happens? 
Everyone rushes the door, and they get stuck on each other as they try to squeeze through all at once.  In extreme cases people get trampled, occasionally fatally, but even if everyone stays on their feet, the chaos amplifies the bottleneck and it takes an even longer time for everyone to get through.

Now consider an equally large crowd, but imagine they consist of a highly trained military company.  When the fire alarm goes off, instead of each individual going straight for the door and attempting to shove each other out of the way, they all immediately form a single file line down the center of the room, each taking their place based on where they started, no one "cuts in line", everyone moves at a quick but controlled pace and never any faster than the person in front of them.
In the second scenario the very last person to go through the door gets through faster than the middle person in the free-for-all scenario.
What has changed?  Each individual is moving a little bit slower, they all give each other a little more space, and no one runs around to the edge of the door to try to squeeze in from the side.  The exact things that people acting as individuals do to try to optimize their own individual escape time are what cause them to get stuck on each other and, paradoxically, means they and everyone else gets out slower.
Researchers have looked at this phenomenon of "more haste, less speed":

The desire for speed overwhelms the desire to avoid collision and the blob people jam up against one another -- just as salt can jam the shaker even though the hole is bigger than the largest grain. The room takes longer to empty even though everyone tries to move faster -- handfuls of people escape in bursts between clogging events.

You can see something analogous on the highways everyday.  Drivers attempt to go as fast as possible at all times, even when there are other cars ahead of them.   Many tend to drive as close as possible to the car ahead, much closer than the recommended 2-3 second rule from drivers-ed class.  When coming up on a line of stopped cars ahead, they will keep a foot on the accelerator as long as they possibly can before hitting the brakes hard just in time to prevent impact.  And any time one lane is temporarily going slightly faster than the one they are in, they pull into it to gain one or two car lengths over those around them. 
The overly aggressive drivers are obvious.
But almost everyone contributes, if to a lesser extent, to the same general phenomenon. 
Say every car is as close as is safe to the car ahead, in every lane, and everyone is moving at a constant rate.  Now what happens if one car wants to change lanes?  Since the cars are all as close as can be already, there is no possible way that the car can change lanes smoothly, because someone is going to have to slow to let them in, and they will have to slow to make the merge.  Now the following cars in both lanes have to brake.  And since the cars behind them were already as close as possible to them, those cars also have to brake.  And since the cars behind them... you get the idea... the stopped cars now travels back through the traffic in a wave. 
Now the same scenario - except the drivers are self-regulating like our military company escaping the burning theater: each car leaves a gap from the car ahead of them large enough that another car can safely merge in front of them.
Now when a car inevitably needs to change lanes, they can do so without slowing down, and without making the car behind them slow down.  They and the car behind them will want to reopen the gap that they just filled, but this can be done gradually over time, with only minor adjustments to speed, and the wave of stopped cars never occurs.

Traffic engineers can control individual driver behavior by putting in deliberate bottlenecks, called metering lights - the kind found at toll plazas and some on-ramps, where everyone is supposed to wait just a couple seconds before they merge with traffic.  Everyone ends up on the highway, but that moment of waiting forces everyone to space themselves out, and even though you had to wait, it is more than made up for by higher average speeds for everyone - including you. 
Sometimes you can even see a similar effect from lane closures or rubbernecking - a section of highway that is moderately backed-up everyday, but on one occasion has a lane closed for construction or due to an accident, if its near the beginning of your trip, occasionally has you get to your destination faster than usual.  As the cars slow down for the bottleneck, and then reach the end and start to accelerate one at a time, they spread themselves out, just like a metering light would have done. 
An identical effect is seen with the crowd of pedestrians trying to get through a doorway; putting an obstacle in the way of the exit actually makes the crowd get though it faster.

Most fire codes require that the pathway to an emergency exit be kept wide open, but according to researchers in Japan, placing an obstruction next to an exit may actually help crowds of people to get out of a room more efficiently.

Researchers found that when people bottleneck near an exit, they start to jostle each other for position. The jostling acts much like friction, slowing down the rate at which people can exit. Introducing a strategically-placed obstacle near the exit can reduce the number of people pushing for the exit, speeding up the rate at which people can pass through.

"We found that we can evacuate faster if we put an obstacle at the suitable position in front of the exit," said Daichi Yanagisawa, who lead the study from the University of Tokyo in Japan.
Even without metering lights, though, you can make a conscious choice to help traffic you are in move more smoothly.
Pay attention to the road ahead.  If you see an ocean of brake lights up ahead, take your foot off the accelerator.  There is no point in racing to be the first to come to a stop.  Resist the urge to change lanes every time one appears to be going slightly faster, unless you have enough space that you can do it without anyone having to slow down for you. Leave a big enough gap between you and the car ahead of you that someone else could safely merge in front of you without you having to slow down.  That applies at any speed, from stop and crawl to over the posted limit** - not only will it smooth out traffic flow, it will also reduce your chances of being involved in a collision, not to mention reduce other people's road rage.  No one can cut you off if you choose to slow down and let them in.
That means people will get it front of you.
And that's ok.
At as slow as 10mph, one car length costs you all of one second.  At 35 it costs you one third of one second.  Big freggin deal!  Let 50 cars get in front of you on a trip with a 45mph average speed, and you get where you are going all of 30 seconds later than you would have had you made sure to be the one to go first.
Not only have you made 50 people a little happier, but you have helped traffic flow a little better for all the people behind you, all the way back down the highway.
Better still, when you are coming up to one of those pointless braking waves, and you start slowing down well in advance, often times it will have completely cleared itself up by the time you get to where it was.  Which means by simply taking your foot off the accelerator, you never have to brake at all.  By avoiding coming to a complete stop, your average speed ends up being higher!  Its quite like timing traffic lights - if you try to go faster than the timed lights are designed for, you have to stop for the red, and someone driving at the speed limit will pass you just as it turns green again while you are accelerating from a stand still.
And if driving with less pointless starting and stopping, less stress, and helping to clear up traffic jams wasn't enough, this also happens to be the best way to minimize fuel when driving in traffic, so you save cash too, along with the environment and America's energy independence. 

Next time you are driving, think about this essay.  When someone exits in front of you, leaving a huge gap between you and the next car, don't rush to catch up.  When you are entering the highway, and the on-ramp is clear but the merging lane is slow, don't stay on the on-ramp until the very last second and then cross over the solid white line in an attempt to pass as many other cars as possible.  You are saving yourself a negligible amount of time, probably less than a second, but you are creating a braking wave that will snarl the traffic behind you potentially for miles.  Think about the orderly single file line, and how much faster everyone exits the building.  Everyone else is going to drive how they are going to drive, but at least you won't be making it worse.  And who knows, if enough of us start doing it, a few others might just take notice, and sooner or later stop and go traffic waves will simply cease to exist.

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