From Longman Dictionary of Contemporary Englishheadroomhead‧room /ˈhed-rʊm, -ruːm/ noun [uncountable] 1 TTCthe amount of space above your head, especially when you are in a car2 TTR British English the amount of space above a vehicle when it is under a bridge
Examples from the Corpus
headroom• Once fully open there is 25m headroom above the river's 30m wide navigation channel.• Legroom is generous, and headroom reasonably good.• Quantities of leg and headroom are generous, rearward seat travel in particular, and all-round vision is terrific.• The floor was often dug out by as much as three feet to increase headroom.• This gives the tape more headroom for high frequencies.• Paradoxically, my own company has always given plenty of headroom, but not everyone perceived it thus.• But once they're there, once you've given them headroom, they seem pretty determined to stick around. 