And after adjusting for age, gender, race and alcohol use, drivers who tested positive for marijuana were no more likely to crash than who had not used any drugs or alcohol prior to driving.
You have to take into account that the population under study -- drivers who tested positive for marijuana -- are people who consumed some amount of marijuana AND judged themselves capable of driving. Some pot smokers will exclude themselves from this population because they estimate that they can no longer drive safely -- myself being one of them. So from this study you cannot conclude that, in general, smoking pot does not affect your driving skills.
Yes you can conclude that precisely because you don't lose the ability to know when you've had too much. Unlike alcohol, pot doesn't affect your ability to judge that.
That's not what that means, though. All that says is that testing positive for marijuana doesn't provide a meaningful measurement of impairment. It does not mean pot doesn't impact your driving.
Everything impacts your driving, unless you can show that driving while high is an actual problem that needs solved (it isn't) then there's no justification for any laws about it.