Most people do not desire to be stopped by a police officer when they are driving their vehicle, for a lot of good reasons, of which, some of the most salient are: they don't want to be shot or killed in cold blood, they don't want to be arrested, they don't want to receive some sort of traffic violation, they don't want their vehicle to be searched, they don't want their vehicle impounded, they don't want their time wasted, they don't want their car insurance rates to go up, and they don't want their stress levels to go sky high. The bottom line is that police officers carry a lethal weapon and further have the power to arrest people, of which these things rightly frighten those that have been pulled over, irrespective of whether they are guilty of anything or not.
Basically, if traffic stops were something akin to receiving a parking ticket, in which a citation is merely issued by an authorized person that is not a police officer; this thus would represent something that though undesired, is not typically going to be life threatening, mainly because the citation is straight and to the point, and the risk of a whole bunch of other possible negative actions occurring, would be virtually nonexistent. On the other hand, police officers are allowed to pull over any vehicle that is perceived to have violated any of the myriad traffic violations, and police officers nearly always can find some code that is relevant to why they are pulling over a particular vehicle, such as "failure to signal for turns or lane changes", or "failure to maintain an assured clear distance ahead," and so on and so forth, of which, whether these violations have even occurred is often not clear. Further, all vehicles are identified with a license plate, of which that license plate identifies the legal owner and the current address on file for that vehicle owner, so that, police officers, with their technology at their beck and call, most certainly have access to the name of the owner of the vehicle, and therefore have a good idea of the probable driver of said vehicle. This thus makes for an opportunity for police officers to deliberately pull over vehicles that interest them, rightly or wrongly.
After all, just because somebody is driving on the public roads of America, does not and should not mean that police officers have carte blanche to pull anybody over, utilizing supposed traffic violations as their ready justified excuse in order to do so, and thereby providing those officers with an opportunity to search the vehicle or the person, under questionable circumstances. To the degree that this is done, reflects that rather than America being a beacon of liberty and freedom, it is in reality, a country in which outright discrimination against certain elements of this society, is permitted, by the policing arm of the state, of which studies have demonstrated as reported by colorlines.com, "... that officers ticket, search and arrest Black and Hispanic drivers more often than Whites."
Nobody wants to be pulled over for a traffic violation to begin with, but when that is combined with a system, that allows police officers, to know who they are more than likely pulling over, and to thereby deliberately target certain individuals to the exclusion of other individuals, that is discrimination. Further to the point, when policing agencies are able to hide behind supposed traffic violations as their reason for their traffic stops and subsequent questioning of the driver of that vehicle, the right of those drivers to be secure and to be treated fairly in their person, has been violated.