In Terry v. Ohio, the U.S. Supreme Court ruled that a person can be stopped and briefly detained by a police officer based on a reasonable suspicion of involvement in a punishable crime.
[7] A brief, non-custodial traffic stop is considered a "seizure" for the purposes of the 4th Amendment and must therefore be supported by reasonable suspicion or probable cause.
The investigating officer must weigh the totality of the circumstances to determine whether sufficient objective facts exist to create reasonable suspicion that the driver is engaged in criminal activity.
If his or her suspicions are confirmed, then there may be probable cause to either search the vehicle and/or arrest its occupant(s), depending on the nature of the suspected violations.
[10] Note that some states also impose additional notice requirements for roadblocks, such as appropriate signage and/or flashing blue or red lights.
A few years after New Jersey v. T. L. O., the Supreme Court held in O'Connor v. Ortega that while government employees do have Fourth Amendment rights in the workplace, administrative investigations conducted by supervisors looking for evidence of work-related misconduct or violations of an employee policy – unlike investigations by law enforcement looking for evidence of criminal offenses – only require reasonable suspicion to justify a search.
[13] The purpose of this training is to enable supervisors to determine whether reasonable suspicion exists to require a driver or other safety-sensitive employee to undergo testing described in §382.307.
However, reasonable suspicion does not apply merely because a person refuses to answer questions, declines to allow a voluntary search, or is of a particular race or ethnicity.