headroom vs headway

headroom and headway both are nouns.

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

As nouns, headroom and headway are synonyms defined as:

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

Other synonyms of headroom include clearance.

headroom (noun)	headway (noun)
the capacity of a system to reproduce loud sounds without distortion	forward movement
vertical space available to allow easy passage under something	vertical space available to allow easy passage under something