This point was illustrated recently in a U.S. Supreme Court decision that was issued last month. The case involved a pickup truck that was pulled over by police by the California Highway Patrol. Troopers initiated the stop based on an anonymous 911 call which claimed that a truck matching the vehicle's description tried to run the caller off the road. The troopers had no independent verification that the truck did anything wrong except the word from the anonymous 911 caller.
When the troopers approached the truck, they detected a smell of marijuana; and then discovered 30 pounds of pot following a search of the vehicle. The two suspects filed a motion to have the evidence suppressed on the grounds that it was obtained from an illegal search. Once their motion was denied, a guilty finding was entered for the crime of transporting marijuana.
A California Court of Appeal affirmed the ruling, and the case was later appealed all the way to the U.S. Supreme Court. In its ruling in Prado Navarette et al. v. California, the high court stated that although an anonymous tip by itself does not always constitute probably cause, the description of the suspected offending vehicle and its behavior was enough to warrant a traffic stop, especially since such driving behavior could be an indicator of DUI. The court also noted that it was reasonable to assume that the tip was valid based on the timeliness of the call in relation to the incident. As a result, the Supreme Court affirmed the lower courts' rulings. This case stands for the the idea that 911 callers are "reliable."
It seems that the "probable cause" bar is getting lower and lower each year. Let us hope that this decision does not spur a wave of "anonymous" 911 calls to authorities in an effort to circumvent probable cause.