The Supreme Court in 1925, ruled in Carroll v. United States that because automobiles are highly mobile, that a warrant is not required to search a given vehicle, if the police have reason to believe that the vehicle contains evidence of a crime, or contraband of a crime. In short, this meant that while driving a vehicle, the right of the people to be secure in their persons and effects, are subject to being violated, without a warrant needing to be issued, simply based on the "probable cause" that a given crime, has been committed, as determined by the police.
This decision, was somewhat remarkable, in the sense that cars were at one time, commonly referred to as "horseless carriages," signifying that there was a time when the normal means of transportation was the utilization of horses, also highly mobile, in addition to the fact that horses could be attached to carriages, making them readily capable of transporting contraband or other goods, so that, at the time of the writing of our Constitution, the mobility of man from one place to another, was often conducted utilizing a form of transportation, that was highly mobile, i.e. horses.
Since this Supreme Court ruling there have been additional rulings, in regards to warrantless searches of automobiles, but fundamentally the Supreme Court decision as determined in 1925, stands as is, which permits warrantless searches of vehicles, based on probable cause as determined by the police, still being the law of the land. The substance of this law as enforced in America, is that, by definition, anytime that a given individual desires to move from one place to another, that person, is subject to being stopped by the police and subject to a search of their vehicle as well as to their own person, all under the guise of "probable cause," of which, the police, can essentially pull over anybody for any reason, under the color of law, so that in effect, "witch hunts" are authorized by this statute, and the Fourth Amendment, for all practical purposes, is null and void.
The upshot, though, is that the Fourth Amendment, could have easily dealt with the issue of the mobility of man, by making it part and principle of its Amendment, of which, it did not. In addition, during any part of a given person's mobility, a person will begin from one destination and arrive at another destination, so that, the police or other governing authority obviously still have the ability and capability to see and to notice all and everything done out in the public view from such transportation, so as to thereby create an actual probable cause of real actionable information, that a specific warrant can therefore be generated upon.
The reasoning behind the automobile exception becoming law, is that because automobiles are mobile, therefore, an exception must be made in order to stop a crime in process, or to interdict contraband, all under the logic that this has to be done, before the vehicle travels away and therefore is able to offload the contraband or to remove all vestiges of a crime that has been committed, but in point of fact, police essentially use the automobile exception, to violate the people's Fourth Amendment rights, under the guise that they need do so in order to stop crime and to punish criminals. However, in practice, what is established is a police state, in which the people are never secure from unreasonable searches and seizures, because the police have not been troubled beforehand to find probable cause and have not been issued a warrant specific to the place and thing to be seized.
The mere act of driving in one's vehicle, the equivalency of riding one's horse, should not now, and should not ever mean the removal of the protection to be free from arbitrary searches conducted by the state against the individual.