headway vs headroom

headway and headroom both are nouns.

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

As nouns, headway and headroom are synonyms defined as:

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

Other synonyms of headway include clearance.

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