I have worked with quite a number of pilots who do not appreciate that there is (was?) a difference between the two.
Given take-off clearance at a downtown airport in the NW of British Isles a while ago with "... and climb straight ahead on the runway heading." There ensued a brief and confused discussion in which I sought clarification - and I don't think either ATC or my F/O really understood what I was talking about.
"Straight ahead" or specify a heading assigned by radar would clarify all.