Steve Chapman writes eloquently  on what are called consent searches:
The other day, the American Civil Liberties Union 
of Illinois issued a report on "consent searches" that sometimes
accompany traffic stops. Relying on data provided by local and state
law enforcement agencies, the report documented that black and Hispanic
drivers are much more likely than whites to suffer such invasions—even
though the cars of minorities are far less likely to yield contraband.
These treasure hunts are called "consent searches" because they require
the motorist to give permission. They take place only when the police
officer has no grounds for suspicion. If he has probable cause, he
doesn’t have to ask. Only when he’s acting out of a vague hunch, racial
prejudice or simple malice does he need the driver’s consent.
But the term is fantastical in these instances. Stopped on a lonesome
stretch of highway, at the mercy of an armed man who has the power to
arrest, very few citizens feel free to refuse. The Illinois State
Police report that 94 percent of white motorists and 96 percent of
minority ones "consent" to such searches.
Is that because
they have nowhere else they’d rather be? Is it because they get a kick
from watching a cop take apart their cars in an effort to put them
behind bars? Or could it be because they suspect that refusing a cop is
far too dangerous?
A conservative would argue that innocent people have nothing to fear. So it’s all upside–you might catch a criminal. The libertarian worries about the power of the state, willing to let some criminals be undetected.