Takes on a double mean­ing: active observation of present-day urban agglomerations and development of hypotheses on the structure of a situationist city.
Guy Debord, Rapport sur la construction des situations
One can dérive alone, but all indications are that the most fruitful numerical arrangement consists of several small groups of two or three people who have reached the same level of awareness, since cross-checking these different groups’ impressions makes it possible to arrive at more objective conclusions. 
Internationale situationniste #2
Our rather anarchic lifestyle and even certain amusements considered dubious that have always been enjoyed among our entourage—slipping by night into houses undergoing demolition, hitchhiking nonstop and without destination through Paris during a transportation strike in the name of adding to the confusion, wandering in subterranean catacombs forbidden to the public, etc.—are expressions of a more general sensibility which is no different from that of the dérive. 
Internationale situationniste #2
Within architecture itself, the taste for dériving tends to promote all sorts of new forms of labyrinths made possible by modern techniques of construction.
Internationale situationniste #2