Surprisingly enough, many people do not use their cruise control.  You may be driving along with plenty of speed to pass another motorist, but as you pull alongside them, they speed up.  One of two things are probably happening.  Either you have lost some speed and you are not aware of it and only think they are speeding up, or they have actually sped up.  It is a very natural tendency to speed up when being passed.  Most people do not do so simply to be antagonistic, but it is easy to believe that they are.  Chances are that if they had been going slowly enough that you had been able to pass them before, they will eventually slow down again. 

On the other hand, sometimes they gas on it and take off and it never seems to fail that a few miles down the road, you catch them again.

Sometimes it is easy to think that someone is playing games with you, but that really is not always the case.  Trucks usually lose at least some speed on hills.  You may be able to pass a car easily on a downhill, but on the uphill, the car will catch you again.  You may find yourself in a passing battle at times.  If you cannot get past them, sometimes it is best to exit the interstate and sit on the ramp for a few minutes, or go ahead and make the stop you were planning to make anyway. 

You will find yourself playing this little “game” with other big trucks just as often, if not more often, since trucks have the additional limitation of a speed governor.