Ummm,,,point out the fact that you bought a responsible bike and not a crotch rocket (trust me,, the parents all think cruisers are slow and not as dangerous,,just go with it),, that you ride responsibly,, point out as stated that freeways are less likely to have you involved in an accident historically versus side streets, and make sure she sees you riding into the area and away from the area carefully.
My Mom was the same way,, she has settled down and I also point out that I have been careful and she is even contemplating letting me take her for a spin on the bike to ease her worries. Haven't done it yet,, but the attitude has gone from no way in hell I would ever get on one of those death machines to well not right now,,maybe later,,, so improvement does happen,, it takes time and you being able to demonstrate you are a safe alert rider. Good Luck on it!!!!
__________________ Biketoberfest 2006 flashback courtesy of Intimid8er: "Like you knew damn well you shouldn't eat something like that, but all be damned to hell, you were gonna eat it!" 
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