He--, Brad, don't ask us! Ask an attorney, and be willing to pay a few bucks for the right (or at least a good) answer. Even if an attorney is interested in this site, why should he give legal advice, free, to someone who is not legally his client. This is not greed, it is important to the attorney-client relationship.
OK, that was pretty abrupt. Generally, with a car they only need reasonable suspicion. I have a lengthy post about this on another thread. The courts have held that the police can hold you for a time (less than an hour) before either getting a warrant or letting you go. The thing is, don't give them a reason to stop you in the first place, and if they do, don't give them reason to search, like AR-15 magazines all over the front seat. Drive right, obey the laws, move with traffic (which may contradict obeying speed laws), keep your car in good condition. Avoid provocative bumper stickers (they can't stop and search for that alone, but a "Kill all cops!" sticker just might give them ideas). No burned out bulbs, wired down trunk lids, broken glass. (A broken window, especially driver's side, screams "stolen car."
Seriously, if you really want to know, ask for a half-hour of an attorney's time. If a friend or relative, you might get it freebie.
[This message has been edited by Jim Keenan (edited April 12, 1999).]