How do you responsibly test something like this? The payload isn't going to follow a ballistic trajectory, it's going to follow an aerodynamic one. You'd probably have to design something special to load into it to aim the damned thing.

Which is a road trip to some place very empty in something like a 180º arc. Maybe 225º.

Very early in the video, he explains that while previous builds used steel ball-bearings, that's not responsible at these energy levels, so he switches to lightweight balls made of biodegradable plastic. There's comparatively little energy in the projectile after release, and it's only supersonic for a few feet.

Further, you test in an enormous empty field, as he's doing.

He has fairly specific mechanically timed release control over it — within 0.5 degrees.

But he also used an extremely lightweight ball projectile — a few grams of 3D printed plastic — because he was only interested in the launch speed. And he pointed it in a direction where there were no people for miles.

He explains that the field he is shooting towards is over 5km of empty field. He also started his tests with much lower weight and speed to ensure proper behavior before getting up to full speed.

It's shooting 3d printed balls, I imagine they don't stay supersonic for very long. Significantly less dangerous to others than driving a car.

One possibility is to mount the trebuchet on a tiltable platform and tilt it to a suitable elevation, say 45 degrees.