In "Cannabis and driving: a review of the literature and commentary", the United Kingdom's Department for Transport reviewed data on cannabis and driving, finding although impaired, "subjects under cannabis treatment appear to perceive that they are indeed impaired. Where they can compensate, they do...". In a review of driving simulator studies, researchers note that "even in those who learn to compensate for a drug's impairing effects, substantial impairment in performance can still be observed under conditions of general task performance (i.e. when no contingencies are present to maintain compensated performance)."