I noticed that too, but I no longer play it. I have moved on to bigger better things and swapped the "Need" for "Live" because in general LFS feels much more realistic. I felt that Need For Speed cars felt plasticy and poor quality, with tacky bodykits an what-not. Anyway, thats a big O/T, so back On Topic.
I suppose they put them there so you had to take more car while throwing your car into a corner, or are you meant to be sloppy and drive up them. I know I love to do it deliberatly, but I don't suppose the underneath really annoys me. Theres more important things surely, like fixing those moronic barriers? Just my opinion though, go ahead and start a debate...