Cattle guards are trenches dug in highways with metal bars across them spaced so that cars can drive across and pedestrians can walk across with no problems but the spacing makes it impossible for cattle to walk across.

See http://www.emporia.edu/english/hoy/cattleg.htm

or

http://photography.cicada.com/gallery/prescott/image02.html

I suppose the cattle could drive across if they were in a Far Side cartoon.