Why? Because they decided that a real scientific study was needed pointing out that "Previous research studies on cannabis and driving have focussed largely on the effects of cannabinoids on driving performance. These studies have been almost exclusively experimental, involving laboratory tasks, driving simulator and on road 'real driving' experiments." In other words, science was, um, scientific, and real experience didn't count.
I found this most amusing: "There was a reduction of average speed on the motorway when participants had the high or low doses of cannabis. This confirms the results from many previous studies. It strongly suggests that the participants as drivers are aware of their impairment, but attempt to compensate by driving more cautiously." The underlining is my addition.
Now, I am no expert on driving while stoned but my first experience of being a passenger in a vehicle driven by someone who was stoned was in 1968. Let's call the driver Randy (because that is his name). First, the vehicle was a 1965 Corvette. But, more important, I had never seen Randy do anything with caution. He was more "out on the edge" of anyone I had met in my life.
We had been at a party. I had been drinking alcohol but not smoking pot. Randy had been doing both but, as the drinking had mostly been early in the evening, I suspect most of the effects of the alcohol (on both of us) had pretty much worn off. On the trip home Randy was driving at maybe 25 mph in an area with a 45 mph speed limit. On the way to the party he had been driving at more like 60 mph.
I (possibly naively) asked him why he was driving so slowly. He explained that things were interesting. He talked about seeing the stars. While stopped at a traffic light he talked about all the interesting lights he could see. No halucinations—he was just talking about the streetlights, lights on houses and such.
My conclusion here is that while scientific studies may tell you what you need to investigate, real-world experience is a lot more useful. And, well, if you have seen Randy recently, tell me how he is doing.