Found something - interesting, but not what I'm looking for. I'll keep searching on the turnaround issue.
From U.S. Supreme Court, DELAWARE v. PROUSE, 440 U.S. 648 (1979), CERTIORARI TO THE SUPREME COURT OF DELAWARE, No. 77-1571. Argued January 17, 1979, decided March 27, 1979.
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR> Except where there is at least articulable and reasonable suspicion that a motorist is unlicensed or that an automobile is not registered, or that either the vehicle or an occupant is otherwise subject to seizure for violation of law, stopping an automobile and detaining the driver in order to check his driver's license and the registration of the automobile are unreasonable under the Fourth Amendment. Pp. 653-663.
(a) Stopping an automobile and detaining its occupants constitute a "seizure" within the meaning of the Fourth and Fourteenth Amendments, even though the purpose of the stop is limited and the resulting detention quite brief. The permissibility of a particular law enforcement practice is judged by balancing its intrusion on the individual's Fourth Amendment interests against its promotion of legitimate governmental interests. Pp. 653-655. [440 U.S. 648, 649]
(b) The State's interest in discretionary spot checks as a means of ensuring the safety of its roadways does not outweigh the resulting intrusion on the privacy and security of the persons detained. Given the physical and psychological intrusion visited upon the occupants of a vehicle by a random stop to check documents, cf. United States v. Brignoni-Ponce, 422 U.S. 873; United States v. Martinez-Fuerte, 428 U.S. 543, the marginal contribution to roadway safety possibly resulting from a system of spot checks cannot justify subjecting every occupant of every vehicle on the roads to a seizure at the unbridled discretion of law enforcement officials. Pp. 655-661.
(c) An individual operating or traveling in an automobile does not lose all reasonable expectation of privacy simply because the automobile and its use are subject to government regulation. People are not shorn of all Fourth Amendment protection when they step from their homes onto the public sidewalk; nor are they shorn of those interests when they step from the sidewalks into their automobiles. Pp. 662-663.
(d) The holding in this case does not preclude Delaware or other States from developing methods for spot checks that involve less intrusion or that do not involve the unconstrained exercise of discretion. Questioning of all oncoming traffic at roadblock-type stops is one possible alternative. Pp. 663.[/quote]
Sounds like they can ask questions, but not search the car? I don't like the strong language used early in the decision being followed by a statement that all oncoming traffic can be stopped.