Separation from parents into adulthood is a highly charged emotional event in many families. Parents model behavior and expect children to follow the example even if it does not fit the particular child because that is all that many parents know to do. Parents often do not want to give up their role as parent because they do not have anything to fill that gap. Is there an uncle or other respected relative or minister or teacher or boss who can intervene on your side?

Long ago in the Stone Age I faced a similar choice. My parents objected to a cross country driving trip from California to Chicago. I just left and found support from grandparents and an uncle and aunt. It was a difficult but valuable learning experience in balancing my thoughts, plans and needs with those of my parents. I learned to see my parents as people with their own fears and hopes for their children. It was a watershed event that only destroyed the old parent-child relationship and replaced it gradually with a more adult one. It helped me learn to rely on my own judgement in decisions.

Are there any alternative ways to get what you want while satisfying and neutralizing your mother's fear? Such as a group ride in a similar direction sponsored by a bike club, a scouting organization,
a church group? You would not have complete freedom but it could be a starting point leading to future adventures on your own. You are in a tough spot but you are acting with more adult flexibility than your parents at this point. Keep searching for an answer.