Like you jump into someone car in a highway or something
If you jump in front of a modern passenger car, they are designed to maximize pedestrian safety. You would probably fly across the hood, smash the windshield, and continue over the roof to fall on the ground behind or to the side of the vehicle. Occasionaly people break through the windshield and end up in the front seat instead, possibly injuring or killing any occupants. If you landed in the road you'd be forcing drivers to swerve and brake to avoid hitting your body, which may or may not be alive at that point. That creates a dangerous situation for everybody on the road.
Jumping in front of a commercial truck with a flat grill, the bigger the better, would be more likely to cause quick death. It would traumatize the driver, and these are working class people just trying to make a living so I think that would be highly unethical.
If you jumped in front of a car you'd also be casuing major damage to the vehicle, potentially totalling it. You'd also be traumatizing the passengers, and you don't have a clue who they might be, it could easily be a family with kids. Severly damaging someone's car and traumatizing them so you can CTB is just plain wrong, there are better ways.
Is it illegal? The details depend on where you live, but where I'm located it could violate numerous laws. It's illegal to be on highways in the lane where traffic drives, and especially illegal on freeways where you're not even allowed to walk on the side median. It could count as reckless endangerment to walk or jump into traffic because it will cause people to try to avoid you which can cause accidents. Reckless endangerment can be a felony charge, so if you survived the attempt you could find yourself doing prison time.