headway vs clearance

headway and clearance both are nouns.

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

As nouns, headway and clearance are synonyms defined as:

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

Other synonyms of headway include headroom.

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