Tuesday, June 23, 2015

Thoughts about traffic

These are thoughts I've had about traffic, basically since I first started thinking about it when I first really experienced it in Philadelphia in the mid-90's.  One thing that has really surprised me is that during the TD 5 Boro bike ride in New York City (Spring 2015), the bicycle density was so high that conditions were comparable to car traffic - and I saw the same patterns of behavior!

  • I've heard average speed / flow velocity decreases with density of cars on the road, and I've anecdotally observed it
  • I've heard & observed that changing lanes causes disturbances that ripple / propagate back from the site of the lane change
    • hypothesis / observation - this effect is caused by the fact that the lane change is in reality cutting off the person behind, causing them to hit their brakes, causing the person behind them to brake, and so on
    • Does the value gained by the person making the lane change exceed the inconvenience to others caused by the lane change?  Highly doubtful that it does, however the person making the lane change will never pay the cost associated with their lane change, and will never connect the fact that when they are cutoff / inconvenienced by someone else's lane change that it's exactly what they've done to others
  • I've observed that when traffic on the highway slows down, more and more people switch to the left lane
    • observed - this increases the density of the left lane relative to the others, causing it to be the slowest!  (I've observed this for many years)
    • observed - above a certain threshold of traffic density, the right lane is the fastest / smoothest moving!
      • (I've spent a good amount of time cruising during heavy traffic - not recklessly weaving - just staying in the far right lane, tracking cars to my left and seeing them they never pass me, and me steadily put distance on them)
    • This appears to be similar to a classic "collective action" problem - everyone individually is just looking at their immediate situation ("The car in front of me is going slower than I want!") and taking the default action ("I'll move left!"), but it's worse because actually individuals could choose to move to a lane to the right and do better, but they don't
  • I've observed that there's no point in merging earlier than the last possible moment.  Everyone collectively might as well as use all the available space.  If you merge early, you're just going to sit in line longer.  What's worse, people behind you will get frustrated, and "leap-frog" if the space is available to do so
  • idea - I've often thought about the mathematical similarity between dense traffic and polymers or solids - individual cars are attracted to each other at far distances (people catch up to the person in front) and repelled at short distances (avoiding collisions).  In the simplest formulation, this is chain of harmonic oscillators, and display similar behavior - propagation of waves (i.e. the ripple / wave effect from a lane change)
  • It appears that the behavior people display in cars and on bikes does not occur when on foot.  For example, the equivalent of traffic when on foot is waiting in line, and people generally don't "cut" in line, or they get called out for it / physically restrained / punished if they do.  However, that is exactly what happens in wheeled traffic, there are long lines trying to get various places and people cut off others all the time.
    • When I had only observed this for cars, I thought that maybe if people could communicate between cars easily - the way you can just talk to someone standing / walking near you - it would improve behavior.  But then I observed the same behavior on bikes, when you could talk to people, and it was no better.
    • I suspect there is a strong social conditioning component to the "polite" behavior we display in lines / on foot, that may have developed over a long period of time, and that given the nature of wheeled traffic, will never develop there

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