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The Caravan Fallacy

Wednesday,  07/23/03  05:11 PM

A while back I suggested Caravans:

What if you built a feature on a car that automatically kept you as close as "safe" to the car in front of you?  {"Safe" would be determined by your current speed and your car's braking ability, and assuming the car in front of you has really good brakes.}  Call this "caravan mode".

Well, the other day I attended traffic school (!) and the instructor told the class the best thing to do in traffic is to relax and leave plenty of room between your car and the next one.  This makes traffic work more smoothly, he went on to explain.  Uh, no.  This being traffic school, of course I did not correct him (nothing is worse than a pissing contest between the instructor and a student in traffic school).  But his fallacy is a common one - perhaps you believe it yourself - and so I wanted to discuss this a bit.

Note: It may be more relaxing and less stressful and more pleasurable for you if you relax in traffic and leave plenty of room between your car and the next one.  That is up to you.  Just don't think that it is "making traffic work more smoothly" and that it is somehow more efficient.  That is the fallacy.

This hits at a crucial phenomenon which requires explanation - why does traffic move slowly?  Why can't everyone just drive 65mph, regardless of the number of cars on the road?  In fact, the speed at which traffic moves is precisely determined by the spacing between the cars.  The more cars in a lane, the closer together they are spaced.  Being spaced close together makes people nervous, so they slow down.  If all cars on the road were welded together at the bumper, everyone could go 65mph and there would not be any problem!

Think of it this way.  Suppose you have a bunch of cars in a lane, all going 65mph.  They are spaced as close as is comfortable at this speed.  Now add a new car to the lane.  What happens?  The car directly behind the new car slows down, because the driver wants to maintain a safe spacing.  What happens next?  The car behind that car also slows down, for the same reason.  In fact all cars behind the new car slow down slightly, to compensate for the extra space it requires.  Now add another new car to the lane.  What happens?  The same thing.  Keep adding cars and the average speed of all the cars behind the insertion point will keep slowing down.  That's traffic, pure and simple.

If you could make it safer for cars to be closer together, drivers wouldn't feel the need to slow down, and more cars could travel faster.  Consider the lane of cars at 65mph again, and say you are the first car behind the insertion point.  And - you have caravan mode.  This means you are following the car ahead at an optimally close distance, the minimum distance which is safe given the braking ability of your car, the speed of the distance sensors, etc.  (So there will be some extra space in the lane.)  Now a car is inserted.  You don't slow down, because you had the extra space.  You simply follow the new car at an optimal distance.  Now another car is inserted.  Same thing happens.  You can see that many cars can be inserted into the lane before finally you have to slow down.  And until you slow down, nobody behind you will either!  This is the beauty of caravan mode, and why it is optimal.

Furthermore, if each of the cars being inserted also had caravan mode, then the lane would be much more efficient.  You could add a lot of cars at 65mph without impacting the existing cars one bit.

Okay, so much for the fallacy.  This is one of those things that sounds great on paper, but it has to be proven.  I must build a prototype!

[ Later: Caravans Revisited - the future is here! ]

[ Much later: Caravans cont - back to the future ]