mm is used constantly in engineering and km will be used for long distances in professional settings.
Those small blue signs at the side of motorways, and the white emergency location posts, both use kilometres.
Yeah, the thing with imperial units and engineering is that once you get anywhere near mass, weight, force, energy, and all the related units for same, it can get very confusing and error prone. So are we talking pounds-mass, pounds-force, or slugs? (The latter being something that probably no one who didn't take engineering/physics/etc. classes has ever heard of.)