If you know nothing about weed, don't make up facts.
There are studies that go both ways but most studies show a link between decreased ability to drive and cannabis impairment and even more a link between combined use of cannabis and alcohol.
The federal government's Department of Transportation conducted research with a fully interactive simulator on the effects of alcohol and marijuana, alone and in combination, on driver-controlled behavior and performance. "The Effects of Alcohol on Driver-Controlled Behavior in a Driving Simulator, Phase I", DOT-HS-806-414. The study found that alcohol consistently and significantly caused impairment -- but that marijuana had only an occasional effect. Further, there was little evidence of interaction between alcohol and marijuana. Finally, speeding tickets and accidents went up with the use of alcohol, but no marijuana or combined alcohol-marijuana influence was noted.
Coming to a different conclusion, however, the California Department of Justice found that marijuana does impair impair driving skills, particularly at high-dose levels or among inexperienced users. "Marijuana and Alcohol: A Driver Performance Study", California Office of Traffic Safety Project No. 087902.
And these studies:
THC affects areas of the brain that control the body’s movements, balance, coordination, memory, and judgment, as well as sensations. Because these effects are multifaceted, more research is required to understand marijuana’s impact on the ability of drivers to react to complex and unpredictable situations. However, we do know that—
A meta-analysis of approximately 60 experimental studies—including laboratory, driving simulator, and on-road experiments—found that behavioral and cognitive skills related to driving performance were impaired in a dose-dependent fashion with increasing THC blood levels.
12 Berghaus G, Sheer N, Schmidt P. Effects of cannabis on psychomotor skills and driving performance–A meta-analysis of experimental studies. In CN Kloeden and AJ McLean (eds.), Proceedings of the 13th International Conference on Alcohol, Drugs and Traffic Safety. Adelaide, Australia: The University of Adelaide, NHMRC Road Accident Research Unit, pp. 403–409, 1995.
Evidence from both real and simulated driving studies indicates that marijuana can negatively affect a driver’s attentiveness, perception of time and speed, and ability to draw on information obtained from past experiences.
A study of over 3,000 fatally injured drivers in Australia showed that when marijuana was present in the blood of the driver, he or she was much more likely to be at fault for the accident. Additionally, the higher the THC concentration, the more likely the driver was to be culpable.
13 Drummer OH, Gerostamoulos J, Batziris H, Chu M, Caplehorn J, Robertson MD, Swann P. The involvement of drugs in drivers of motor vehicles killed in Australian road traffic crashes. Accid Anal Prev 36(2):239–248, 2004.
Research shows that impairment increases significantly when marijuana use is combined with alcohol. Studies have found that many drivers who test positive for alcohol also test positive for THC, making it clear that drinking and drugged driving are often linked behaviors.
National Highway Traffic Safety Administration. Marijuana and alcohol combined severely impede driving performance. Ann Emer Med 35(4):398–399, 2000.