Hmmm, Very interesting. I applaud your ambition, but do think this is a terrible idea. I have hitchhiked extensively, probably 100+ times. Yes, many (most) have been very short rides, but I had done some long distance hitchhiking as well. I hated it. You are truly putting your life and belongings in a stranger's hands. This works well most of the time, but it is always a gamble. All of my experiences have been good, and usually interesting. In the high desert in CA I got a ride where the guy straight up told me that if I was a weirdo and tried anything he would shoot me with his .44 and bury me in the desert. This wasn't a threat, just a general comment. We had a fine ride for about 45 minutes, talking about motorcycles the entire way.
Other times I have been picked up by drunks in the South, people who had lots of dogs in the car that I had to squeeze around, riding in the trunk area of SUVs, etc etc. You will never get a ride from people who have kids in the car. This is a smart decision on the parents' part. Also, it is almost always the crappy cars that pick you up. BMWs and Mercedes practically never stop.
You will hopefully meet many fine people, many interesting people, and no dangerous or terrible people. I really do think 18 is too young for this, and that you do not have enough knowledge of the world or other types of people. Have you ever met someone who lives in the deep south, has never been out of the state, and has a personal meth lab? Hopefully you never will.
I wish you the best on your journey, but will caution you to not let pride get in the way of practicality. If things are not going well, or you are not having fun, just quit. There is no shame in quitting this, absolutely none. Do not carry a lot of electronics on you. ALWAYS hide emergency cash somewhere on your person. I often hid most of my cash under the footbeds in my shoes. Make certain you carry enough cash in your wallet or pocket to make the robber content. The last thing you want is for someone to rob you at gun or knifepoint and not find any cash. If you are robbed of all of your worldly possessions in a very sketchy area, without cash you will be in very serious trouble. Don't be too proud to book a hotel room if you find yourself in the dark in a TERRIBLE neighborhood. There are two nights in particular I remember. I vowed to never repeat them, and have since booked cheap hotels when I landed in a bad spot at dark.
I will continue to hitchhike for convenience, such as back to my car after a lengthy hike or climb, but will hopefully never be forced to hitchhike again. It really is not enjoyable, and I hate being at the mercy of the world. There are a lot of questionable people out there. A lot of older men would very much enjoy meeting a fine young man like yourself. You would not enjoy meeting them.