... right before he stops, you can hear the sound of a beeping car horn, like a signal
to the skateboarder to stop.

IMO it sounded very much like a car horn, which would make sense to have both a driver and a cameraman in the passenger seat to have been following him the whole way down.

It also would've been helpful to have such a close following vehicle with lights on in the tunnels, to help the skateboarder see better and to alert any possible oncoming vehicles in the darkness know to stay snug in their lane.

Made it a less dangerous

... how the hell do you survive practicing something like this, to acquire the necessary skills and reflexes ?

That was nuts. My legs would be absolute jelly if I attempted that.

Does anyone know what highway? Million Dollar Highway maybe?

So I read through the comments, and the one thing that strikes me is how unanimous the praise for both his skill and how this is the most amazing video on YouTube

I don't think I've ever seen comments to a YouTube that didn't have at least a couple of detractors lol

Here is a post related to your question...

"... the sign says "Haslital" so I guess it is in Switzerland.
Another commenter says Sustenpass Switzerland.
I was sure it was New Zealand it is so beautiful, but nope - Switzerland is beautiful as well."