"Henry Jones" wrote
I blocked the road with cones and turned back all members of the public
yesterday, but it was extremely boring, so I can't wait for the signs to
come in. That's for the advice.

Most people said they didn't know it was a private drive, even though there
was one sign saying so at the entrance.

The drive is 18 feet wide so I can make large letters.

I'm thinking of putting down a big "stencil" on the road of something like:
"PRIVATE DRIVE | NO PULIC ACCESS"

Where do you think those stencils would be found?

I think you have to be clear and to the point. Otherwise curiosity seekers and no-goods will ignore signs. Check with the local police for their advice on words. Maybe.....

Private Driveway ("Drive" implies it's not a residential road)
No Trespassing
City Code 12345 (Small letters)