One possible origin of this idea is the fact that some (but not all) studies with airline pilots have shown a modest decrease in capabilities on a flight simulator up to 24 hours after use of one fairly strong marijuana cigarette. In two of the four studies (one of which was not placebo-controlled) the ability of the pilots to safely navigate a simulated aircraft was apparently compromised somewhat up to 24 hours later, while the other two did not show any impairment beyond 4–8 hours. Despite the inconsistent replication, these studies have led some experts to recommend drug testing of people in safety-sensitive jobs (pilots, bus drivers, etc.). In contrast, no "driving" studies have found any significant cannabis-related impairments lasting beyond 2–6 hours.