Road Map (two words)
Two simple nouns, separated by a space, indicating “a map, showing the roads in a region”. Thus, a navigation tool for driving, showing roads, routes, directions, and other details of an area or terrain. See Roadmap, below.

Roadmap (one word)
A compound noun. Combines the two separate nouns of “a road, or path leading somewhere” with “a set of guidelines, instructions, and/or plans”. Creates the single idea of a directive for future development. Thus, a strategic planning tool for the development of projects, initiatives, organizations, and enterprises. Contrast with Road Map, above.