This has me scratching my head!! As babygoose says, out here in ranch country, the 'cattleguard' is pretty common..where a road crosses a fence line, you dig down a couple of feet or so across the road, then install, over that 'pit", something that is like a group of parallel metal bars that run at right angles to the road,as:
________lllll__________
_______ lllll__________ (imagine that there's no 'break' in the groups of 5 closely-spaced lines)
lllll
I have seen them made out of pieces of railroad track; they are spaced a few inches apart--they are usually installed as a single welded-together 'unit'(HEAVY, as you might imagine!) Often, at each end there is a welded, inverted "/\" of pipe,and the fence is attached to/goes on from that. There will always be a gate at one end of a cattle guard, to accomodate riders/others who can't drive over it on wheels! Cattle aren't likely to try to cross a cattle guard, as they fear losing their footing in it(as indeed, they would!)--horses usually won't try to cross, either, BTW--unless they figure out how to JUMP it! (Cattle aren't much for jumping, at least WIDTH!)
In a lot of places in this flat old country out here, there may be a 'build-up' of the soil-a kind of 'berm'-where a cattle guard is installed, for a couple of reasons--so when it DOES rain, that the 'hole' under the CG won't fill up and flood, but the water can 'drain away', OR, to help deter rainwater from 'running' endlessly across the 'flat' and causing erosion. So maybe, THIS is what your friends refer to?? I have NEVER heard of a 'cattle bump'....! I can testify that if you drive across most cattleguards too fast, you WILL feel quite a BUMP, though!
Margo(who has spent well-enjoyed time horseback, gathering cattle across ranch country, and encountering LOTS of cattleguards!)