clearance vs headway

clearance and headway both are nouns.

Word	Noun	Adjective	Verb	Adverb
clearance	Yes	No	No	No
headway	Yes	No	No	No

As nouns, clearance and headway are synonyms defined as:

clearance and headway: vertical space available to allow easy passage under something

Other synonyms of clearance include headroom.

clearance (noun)	headway (noun)
permission to proceed	forward movement
the distance by which one thing clears another; the space between them	vertical space available to allow easy passage under something
vertical space available to allow easy passage under something