This question here raises a sub-question, to avoid accidents. I am looking for some safe ways to practice jumping over cars. I use SPD shoes and disc brakes because I have noticed that I can burst very fast with them. I have no idea whether it is possible to jump over cars easily without SPD shoes and disc brakes. So any good ideas for safe practising?
More useful for you would be going along to a basic aikido course, or if that's not available judo, gymnastics, ninjitsu or some other form that teaches you how to roll. Forget the holds and throws and just learn how to roll. When you can confidently run full speed at a waist-high obstacle and dive-roll over it, you'll be in a better position to survive a bicycle-enhanced version of that.
Find a grassy children's playground with a suitable step, bank or ditch, and practice going over the handlebars. As freheit suggested in comments, wear protective equipment. I suggest starting slowly and using an old BMX rather than a bike you care about. The bmx will be easier to get off as well as cheaper.
To be honest this isn't something I would practice. What I get people to do when I'm training them is just turning the bike to one side and fall away from it. At walking speed. The idea is just to convince them that they're not going to die if they fall off, and to give them a few tips on keeping their arms bent. Mostly I use it to sell them on the idea of cycling gloves (and not riding in crappy footwear). Note that I've only done this where helmets are mandatory, but otherwise I'd also be selling people on helmets.