Your design vehicle is a bit larger and at a steeper angle than used in guiderail tests. That means the barrier deflection will be more than is shown on the table. Still, it may help.
If you said what kind of pipeline it is, I missed it. If it's natural gas or some other hazardous substance...
I'm assuming it is solid polystyrene pieces, not styrofoam or packing peanuts. Those would settle under the weight of the Type C material above them. If you need a lightweight aggregate, expanded shale might be more durable.
I agree with Bridgesmith about the deflection distance of the rail...
