He wasn't impaired, per the police officer at the scene.
It's a weird story, but let's reserve judgement until more details emerge. I think Denard Robinson has earned that much (especially considering your point that he was always on the up and up while in Ann Arbor).
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"I don't get sick. I don't observe major holidays. I'm a jackhammer." - Jim Harbaugh & Dwight Schrute