I can think of five factors to consider: How fast are you going, how fast you stop, what you hit, how much you weigh, how/where you are seated.
Obviously speed is a factor. But also how fast you stop. If you hit a giant pillow, it will absorb all the impact and you will be fine. If you hit a tree (which you did) the tree will dig into the car, and the car's crumple zones will absorb the impact. But if you slammed head on with another car and came to an instantaneous stop, the crumple zones on your car no longer work, and now the full force of the impact is fully transferred to the occupants.
Also consider a big ping-pong ball hitting a window compared to a baseball. If the speed of impact is the same, the ping-pong ball with less momentum will bounce off. The baseball will crash right through the window. Same thing with a person. The car's speed is the same, but the heavier person will fly further, and hit harder than a light person.
Lastly, where is the person seated. If the steering wheel blocks a person's path, or an arm gets caught behind the seat, etc. then the person will not go far. But if the car dipped, and gave a person a clear path straight to the windshield, then there's nothing to stop you.
I saw a show once that demonstrated an instant stop from 10 miles per hour. Everyone slid forward, out of their seats. With brakes, suspension, and seat motion, it's impossible to demonstrate that stop with a car... but step up the speed and those factors play less and less of a role. I also know that under full braking force, my car can stop fast than I can. With full brakes, the only thing keeping me seated is a locked seatbelt, and a locked leg (up against the pedal/footrest). Otherwise I would fly forward.
That being said, if all the worse case conditions were met, I would think 50km/h (30mph) would be ample to send a person through the windshield. A typical crash, with average conditions would probably require a higher speed... perhaps 50mph (80km/h). I think by that point, flying through the windshield is one of many injuries you would receive...
Just keep in mind, kinetic energy, or rather, force related to velocity... the velocity is squared. So even a small increase in speed results in a significant increase in force.