Christopher's Law states:

A car driving at a constant speed on the highway will begin to accelerate when another car enters the highway next to or slightly ahead of the subject car.  The subject car will continue to accelerate, above the speed limit as well as what the driver feels is safe, until they have past the merging car. Sometimes the subject car will slow down prior to passing the merging car if the speed has become excessive.  At which time the subject car will immediate realize how fast they are driving and immediately resume previous constant speed.

So this is the observation I have made many times over the last 20+ years of driving.  I discovered it back in 1988.  I'm not sure about why this happens but it does.  Maybe a psychologist can explain it and maybe they already have, in which case I guess I could no longer call it "Christopher's Law".  There are probably several factors but I speculate it has something to do with factors like competition and ownership.  Ownership; The driver may feel since they are already on the highway that they "own" this spot in the traffic flow and will defend that spot by accelerating to maintain ownership.

NOTE: DO NOT TEST THIS! Testing this "Law" would require breaking the Law!  It requires driving above the speed limit (speeding) which is not only illegal but dangerous.  DO NOT TEST THIS!