Airspace

Airspace means that space located above and/or below a highway or other transportation facility's established grade line, lying within the horizontal limits of the approved right-of-way or project boundaries.

Source

23 CFR § 710.105

Scoping language

Terms defined in 49 CFR part 24, and 23 CFR part 1 have the same meaning where used in this part, except as modified in this section.