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Since it's March it's very possible you could run into snow over the passes if a storm blows through. McKenzie Pass won't be open this early but Santiam Pass will. All the highways are plowed but if a storm occurs you might have to wait it out.
Also, if you are planning to camp, many campgrounds won't be open in March so plan your overnights accordingly. And you won't have the long daylight hours like you do in the summer months.