Kim du Toit has a link to a story about a Brit who was asked if the cops could search his vehicle for 'training purposes'. They arrested him for having a Swiss Army knife in his case. That's in Britain, where owning damn near anything is illegal anymore, but similar things happen here.
A lot of agencies/cops are real big on "Do you have any guns/drugs/whatever in your car? Can I search it?". If you say yes, anything- ANYTHING- they may find can be used against you. A lot of them don't care if you're traveling through, and your offense is not having a legal firearm cased properly according to local/state law, they'll bust you.
And if you say no...
I've heard far too many stories- including from people in the law enforcement community- about how they bully/threaten people to push them into saying yes. And they can make you sit on the side of the road for a time while they summon a drug dog to sniff around the car, and if he alerts on anything- including prescription meds- they'll use that as cause to take the car apart. And from what I've heard, they'll use the various criminal history databases in ways they're not supposed to be used to try to find 'cause' for a search.
There's far too many people with badges out there who have gone completely into the "us vs. them" attitude toward EVERYONE; and it's costing all the way around. The cops look with distrust/contempt on anyone not a member of the club; citazens treated with obvious contempt tend to return it; and someone who gets their vehicle taken apart because they dared say 'no' to a search are going to have a bad attitude toward law enforcement ever after in many cases. Doesn't do to tell them 'it was just one jerk', because after that the attitude of other cops that they may have glossed over in the past now becomes obvious and resented.
In some ways the Patriot Act has made it worse. Yes, I know there's no record of it being actually abused(that I know of), but the fact that some agencies can do damn near anything and blanket it under that- even theoretically- really pisses a lot of people off. The actions of a lot of the TSA people at airports is a good example; they can do something really egregious, and if you object you can be arrested. Doesn't matter that they were wrong, WE'RE THE TSA, PEOPLE, and you better put up with anything we do or else! And it is abused, and people rightly resent it, and that takes things further downhill.
Personally, I would never agree to a search of my vehicle. If they think they have legal reason to search my vehicle- or house, for that matter- they can damn well get a search warrant. I know I'm not a legal scholar, and there's so many complications in this/one of the damn problems/ that there's probably ways they can search without a warrant. But I'll not agree to help them, and if they go outside the bounds I will file a complaint, or get a lawyer if I think it's warranted.
I keep remembering Martha Stewart went to prison for "lying to a federal investigator". She wasn't under oath, wasn't in a courtroom, but they were able to convince a jury that she 'lied' to an investigator in an informal interview about a crime they can't prove she committed.
Ain't all this wonderful?