Collision Avoidance

In networking, there is this idea called collision avoidance.  In data networks, information is sometimes transmitted over a medium that is shared by multiple computers.  In these circumstances, it is possible that two computers will start transmitting data almost simultaneously, corrupting both data sets.  Even if both check to make sure no one else is transmitting, one might start transmitting after the other one, but before it detects the signal.  While this is not very common on physical wires since the advent of Ethernet, where hubs, switches, and routers direct the traffic on shared cables, this is still a major concern with wireless networks.  While collision detection helps computers know when corrupted data needs to be retransmitted, the ideal solution is collision avoidance, which prevents collisions altogether.  Collision avoidance involves communication between computers before sending data, to prevent collisions that would waste network bandwidth.

Collision avoidance is also important outside of the field of networking, and most people use it daily.  Maybe you are walking down the hall at work, and you move slightly to the right to avoid walking into another person.  At dinner, two people might try to pass condiments to the same person, and one will have to stop and wait for the other.  Often this is a perfectly smooth exchange where no communication is necessary.  One person gives right-of-way to the other, or both people take measures to avoid collision.  Other times, two people might both move in the same direction, maintaining their course of collision with reference to each other, and then some kind of communication must occur (usually a glance and some laughter).  It is subtle, but the information communicated in the glance will usually make it clear who is going to do what to avoid the impending collision.

Perhaps one of the most common daily uses of collision avoidance occurs while driving cars.  In general, if we follow all traffic laws, we will avoid collisions, but there are some cases where this is not true.  The most notable one is where two people approach a 4-way intersection from opposite sides, and stop simultaneously.  If they do not stop at the same time, the law says the rule is first-come-first-served.  If they are not opposite from each other, the law says the guy to the right gets to go first.  In a situation where there is complete symmetry, however, there is no rule sufficient to prevent collision.  In fact, there is not even a way of distinguishing which car is which (even if you say ,"the bigger vehicle goes first," what if they are the same make, model, year, color, etc...).  This is one case where communication is necessary for collision avoidance.  Sometimes one car will communicate its intent to go first by inching forward.  This does not always work, as both may try this tactic, but it often does.  By far, the most common communication I have seen is the wave.  One car will wave the other to go first.  Whoever waves first goes last.  For this situation, this form of collision avoidance is pretty good.  The wave is a common collision avoidance communication for almost all instances where collision avoidance is necessary in traffic.

Now, here is what I actually want to talk about: the misuse of collision avoidance.  Sadly, this is almost as common as using collision avoidance legitimately in traffic, but instead it actually increases the risk of collision.  Sometimes, at an intersection, one car will clearly stop before the other (in fact, I deliberately regulate my speed when approaching intersections to make sure it is clear who stopped first).  When this happens, the law states that the first car to stop goes first.  Sometimes though, the guy who stopped first will "be polite" and wave the other driver on.  This results in a dilemma.  The waver has essentially signaled that he is forfeiting his right-of-way.  Unfortunately, the law does not recognize this, and it still states that the first to stop is the first to go.  Now, assuming there are no extenuating circumstances (maybe the waver's car just stalled, and he cannot get it to start again), the second driver has to decide to either break the law or wait until the other driver gives up and goes.  This can, and has, caused accidents, as both drivers give up waiting at the same time and crash right into each other.  The worst part is that the fault is legally attributed to the second driver, even though the first told her to go!

Far from polite, waving someone on when it is not their turn is rude and dangerous, not to mention invalid.  If you have the right-of-way take it.  If there is some reason you cannot, then you should probably turn your hazard lights on.  The only time collision avoidance is valid is when there is enough ambiguity that there is an actual risk of a collision.  If you really do not want to go first, the start slowing down sooner, so you do not stop first.  Do not tell people to break the law.  Do not waste other people's time by waiting for them to go when you clearly have the right-of-way. Certainly do not endanger other people's well being in a lame attempt to be polite.