They also have the hard plastic type that secure to the roadway without having to pile asphalt on the road. Plus they seem to have an angled approach and drop on them,a little smoother on you.
The damn plastic ones we have around here are steep angled so when your car comes off it's about the same jolt as a potholt gives you.
They do get the job done though.!!! Before I put larger wheels and tires on the truck I would bottom out the front spoiler everytime.