I wouldn't mention it...... you are already working on "stranger danger" so that is probably enough on that part of the topic.
I would just tell her (as you already have) that you have rules about what she can do... and if she chooses to ignore the rules and ride her bike without you there, she will loose the privilege of riding her bike for a while all together!
And..... unfortunately, all too often, things have bad consequences..... a little boy (about 6 years old, I think) in a small town (population 1,000) we lived in about 30 years ago got killed when riding his bike..... he rode it out from his driveway, and was blocked from easy view by a parked car..... the driver couldn't see him until it was too late! Very tragic ending...... (I knew the mother several years after that incident.... so sad....)
Independence can be a great virtue... at a certain point! Keep up the good work, M., and stick to your rules!