<p>Does have an effect on drivng as it alters both your reaction time and depth perception. No, it’s not as drastic as alcohol or other drugs, but it still places you an the others around you at that risk. This is part of my field of study so I did have to chime in! 
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<p>To your roommate, since you describe her as being nice and easy to get along with, she should understand that you do show some kind of concern. Honestly it is a trick subject and since the risk isn’t very high, I wouldn’t push the issue if she does take your concern offensively.</p>