"Consent" Searches

I've always wondered why somebody who knows he has drugs or other contraband in his car "consents" to allow a police officer to search his car in connection with a traffic stop. Most such searches are the result of intimidation on the part of the officer, with people who aren't sure the cop doesn't have the right to search his car anyway. The fact is, if the cop actually HAS "probable cause," he doesn't NEED to ask for permission. I don't care how many times he tells the citizen that refusal is "evidence" that he has some kind of contraband and that his refusal will GIVE the cop probable cause. Not so. That's a lie. If the citizen refuses, the cop MUST allow him to go on his way, unless he DOES have probable cause, in which case he needn't ask. To intimidate a citizen in this way is the worst kind of abuse, and any cop guilty of it should be FIRED, on the first occasion. I remember one time a cop demanded my son's Social Security number and told him he'd have to hold him for identification if he refused. My son told him that was only in his imagination, that his driver's license is sufficient identification, and that he had no power to compel him to divulge his SSN, and the cop "backpedaled" because he knew he could not make such a thing stick. (Chicago Tribune)