Perhaps you should ask the question "What could go wrong with this system"?

Could it corrode, carry unwanted electrical noise/current, be crushed by large vehicles?

Me, I'd run a 50mm plastic conduit under the road, then run a plastic airline inside.

I would not run inst. air in plastic pipe. Run in ss pipe with di-electric fittings. Also wrap pipe with islolating tape. bury with clean sand, no rocks. compressed air and plastic pipe = a bomb.

There should be no problem running the line underground. Determine why the client wants all instr. air above ground. Does this aply to all services. You will need to run a larger sleeve first and then run the line through it. I would suggest a 100mm minumum and I like the PVC suggusted above.

Line should be sloped to a drain point which will require a sump at the lowest level that the line runs under the road. May be the reason the client wants all instr. air above ground if they have problems keeping the air dry.

Always run a larger line than you actually need their is never enough instr air.

The OP is clear: the air goes through 1/2" carbon steel pipe. The plastic is to protect the pipe and don't need to be tight closed so no bomb danger at all.

I agree with the protecting sleeve (plastic or not) and see no problem in burying the instrument line. Just one point: I would take some measures to avoid difficulties in air leakage case: Do the underground line in one continuous piece (no threaded fittings...), make some access hatches in both sides of the road to allow easy repair...

It depends upon the design life and the specific ambient conditions.

Kind regards