In what circumstances would the police officer have reasonable suspicion that conduct was consistent with criminal activity?

"In an earlier day before the traffic in illicit drugs [and criminal street gangs] became endemic to our society, the above circumstances might notand probably would nothave been consistent with criminal activity. Unfortunately, times have changed, and a multitude of controlled substances [and firearms] are openly bought and sold on our streets and in our neighborhoods. To any police officer knowledgeable of such conditions, the circumstances above described would not only raise, but almost compel, a reasonable suspicion that conduct 'consistent with criminal activity' was taking place." (People v. Brown (1990) 216 Cal.App.3d 1442, 1450, 265 Cal.Rptr. 552.)