The answer is it depends. Looking at case law dealing with a delay at a green light or intersection in North Carolina it appears to be a very fact specific inquiry. Police only need to have reasonable suspicion to initiate a traffic stop and courts will examine all the surrounding circumstances to determine if the stop was justified.
In State v. Roberson, 163 N.C. App. 129 (2004), a police officer initiated a traffic stop when the defendant had an 8 to 10 second delay at a traffic light at 4:30 a.m. The defendant was in an area that had several bars and restaurants that served alcohol, however those establishments were not allowed to serve alcohol after 2:00 a.m. by North Carolina law. In this case the court held that the officer did not have reasonable suspicion to stop the vehicle. However in State v. Barnard, 184 N.C. App. 25 (2007), a police officer observed a vehicle stay stopped at a light for 30 seconds after it turned green. The area was in a high crime area where a number of bars were located. In this case it was held that the officer did have reasonable suspicion to justify the traffic stop. In another case, State v. Parker, 137 N.C. App. 590 (2000), reasonable suspicion was found where a defendant remained stopped at an intersection for 30 seconds with another apparent reason for doing so.
So police may stop you for a delayed reaction at a traffic light or intersection. It appears there must be a time delay of at least a few seconds, which it appears 30 seconds is enough and anything less might be enough depending on the circumstances. Also the surrounding circumstances must be such that may suggest the driver could be intoxicated or there is no reason for the delay. Exactly what would amount to reasonable suspicion in this area is uncertain, but if police can articulate a justified reason you may be stopped for a delayed reaction at a light or intersection.
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