Unless you are ill in some way and really need extra protection what you have described sounds to me a bit like child abuse.
Wow, didn't pull any punches here, did ya? Quite a speculation based on just a few lines in a forum post. And, yes, I did read the rest of your post but still think that using the word "abuse" was waaaaaaay premature.
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to the OP:
I believe that you need to sit down and have a discussion with your parents about this issue.
Pertinent points:
Tell them you want to be an independent fully functional self sufficient member of society -- and that learning how to be on your own is a necessary part of that.
Tell them that just because you are independent, fully functional, and self sufficient, that certainly doesn't mean that you won't ever need your parents ever again or that you won't ever visit or look to them for advice.
You could always try and play the reverse on them: tell them that they too could have a heart attack at any time, and that someone should be with them at all times too. In all likelihood, they probably don't want someone around them 100% of the time, either -- most us need some alone time every once in a while. But, you can show them how silly the rule can be if you try to turn it around on them.
Finally, ask them what additional responsibilities you can take on in order to earn additional freedoms and perks. That is, come up with a list of what chores and other household responsibilities that you will do on time and correctly every time in order to show that you can handle the responsibility of going to friends houses, cycling, and running on your own. In other words, you show your character in that by doing your chores on time and doing quality work, that you are mature enough to handle being on your own. Furthermore, propose solutions to worry about being on your own -- such as carrying a cell phone with e GPS functionality that will allow your parents to see your location at all times. Or promising (and them following through!) to call at certain intervals or when you arrive at your destinations.
The flip side, if you aren't willing to take on additional responsibility, or you've failed in the past to fulfill your responsibilities on time and up to sufficient quality of work, then I feel that your parents are probably rather justified in limiting your perks.
Obviously, judging what is really going on from behind a computer over the Internet over a forum is exceptionally tough. But, usually, you'll do better if you sit down and rationally explain your point of view and make fair proposals than getting emotional and irrational about the issue.