This study does not study what you probably assumed that it study.
The study you expect is. How is the likelihood of a crash altered by using MJ. This study actually studies those involved in crashes and looks at THC, alcohol and other drugs. How many people taking MJ simply avoid driving completely? If 1 out of 2 avoid driving in this condition, this study under reports the accident risk by a factor of 2.
It does refer to several studies that measure impairment based on MJ usage and comments that the impairment in clearly seen in a number of ways.
Also, some THC is not the best measure of the effect of THC. Maybe moderate use has a relaxing effect that actually improves driving by reducing risk taking behavior even if it simultaneous impairs reaction time and judgment while more THC allows the negative effects to outweigh the positive.You would never know in this study.
Given that MJ usage is so widespread, an honest and comprehensive study would be a lot more useful than studies like this one. since MJ is legal now in some places, why not have a real study with various ages, levels of THC, etc. and spend some time in driving simulators. By and large I know people will take recreational drugs and it does not affect me. But I would like to keep them off the roads if this means they are driving while impaired.