The area you placed boulders does not belong to you so I would be careful if I were you.
If the boulders were not there he may, perhaps, have been able to gain control of his vehicle.
I am assuming that neither you nor your witness are experts so although you could say he looked like he was going to fast, you couldn't say how fast!
Is the strip of land registered? Unless you have fenced off this strip to exclude everyone and the owner then you may well fail in your attempt to claim the land. Even so, if you could claim it you may still not be able to do anything with it. As part of the lane it may have a row over it.
Anyway I digress. As he has admitted he was going too fast and lost control you may be able to claim from him. It is worth try as he is ultimately in the wrong.