Following the law regarding "probable cause" can be outweighed by the gravity of the circumstances involved -- a higher priority than strictly following the letter of the law can occur.
For example (to be as extreme as I can simply to illustrate the point): If the police were alerted that white van was carrying a suitcase nuke into Washington D.C. for detonation, you would see a circumstance on the freeways and highways that would make you think last night was mere child's play. No consideration of "rights" whatsoever would be given and we would probably see far more destructive pre-emptive actions taken.
The seriousness of situation can dictate the actions relative to the preservation of "rights" in these circumstances.