My parents had the same response back in 1973 when I was planning a ride with 3 buddies around Lake Michigan. My plan then - which eventually worked - was to not argue, but just keep saying how much I was looking forward to the trip, & kept all the planning with my friends out in the open. I think eventually they (Mother especially) caught on that I was going to go with or without their blessing, & they became reluctantly supportive (Mom even finished sewing my Frostline Kit panniers - which will mean nothing to you, O.P., but does to us Tourers Of A Certain Age.) Funny thing was, if the parents really HAD demanded at the end that I not go, I probably would have caved, not wanting to risk the same sort of estrangement that you fear.

I'd try to remember: the reason your mother is opposed is not just to piss you off, but because she loves you & cares for your safety. In the course of your discussions with her (as per the above posts), it may help for you to verbally acknowledge this to her.

And whatever happens, you need to remember this episode when your college-age daughter tells you she's going to Sweden for the summer to chill with her boyfriend - although it's not really making me feel better....