Traffic stop based on anonymous 911 tip complied with the Fourth Amendment because the tip was sufficiently reliable and created a reasonable suspicion of drunk driving. An anonymous 911 caller reported that a pickup truck ran the caller off the road. The caller also provided the truck’s license plate number, location, and the direction the truck was driving. Based on the tip, CHP officers pulled defendants’ truck over. During the stop, officers smelled marijuana. A search of the truck revealed 30 pounds of marijuana and defendants were arrested. They moved to suppress the evidence, arguing that the stop violated the Fourth Amendment because the officers did not have reasonable suspicion that the defendants were engaged in criminal activity. The motion was denied and defendants pleaded guilty to transporting marijuana. The Court of Appeal affirmed, the California Supreme Court denied review, and the U.S. Supreme Court granted certiorari. Held: Affirmed. Under appropriate circumstances, an anonymous tip may be sufficiently reliable to provide reasonable suspicion to make an investigatory stop. Here, under the totality of the circumstances, the tip was sufficiently reliable and created a reasonable suspicion of drunk driving. The caller claimed eyewitness knowledge of the alleged dangerous driving, which lends significant support to a tip’s reliability. The evidence suggested that the caller reported the incident soon after she was run off the road. Such contemporaneous reports and statements relating to startling events made under the stress of excitement are especially reliable. The caller’s use of the 911 system also indicates that the tip was reliable because 911 callers may be identified and traced, which discourages false reports. Finally, the act of running another car off the road is a significant indicator of drunk driving.