Mythbusters did a test involving not cattle guards, but washboard roads, to test the effects of speed. They built an apparatus that was very much like a cattle guard (a grid of 4 in. angle iron laid 8 in. apart) and drove over it at different speeds with a barrel of water to test for sloshing. Here are the notes from the test:
5mph: “that’s pretty bumpy” – Grant.
40mph: The high speed cameras showed that the wheels skimmed across top of bumps, Grant felt it was smoother. More water was lost, though.
70mph: The high speed cameras showed that the wheels again skimmed across the top, and Grant again felt it was smooth. Less water was lost.
Driving faster on a rough road will give you a smoother ride. (end quote)