they get slicker than - well You know
Snot on a doornob?

For those who are mystified, cattle guards are basically pits or ditches dug across the road, with a series of rails, (old railroad rails, or pipe, or, um 'most anything that will hold up a tractor) across it. Cattle won't cross, horses sometimes get stuck in 'me, and you can drive, or ride across with a minimum of inconvienence. MUCH easier than a gate.

Ah, the memories...