On my grandfather's ranch, he had those rows of pipes laid in the ground with spaces between them at the road entrance. The cows couldn't walk across them, but your car would do just fine.

When he made a back entrance, he just painted white lines instead of the more expensive pipe setup. He'd learned from neighboring ranches that once the cows understand the pipes, they won't walk across the painted lines. And, I don't quite know how, but new cows learn from the existing herd not to try the white lines.