Short answer: both are ‘correct’ but usage seems to be shifting in favour of the single-word form.
I’m updating this post almost a year to the day after I first posted it. And guess what? That tired old cliché of roadmaps is still haunting us. If I go to the UK government website, I find this statement: ‘Planned easements at Step 3 of the Roadmap to go ahead from 17 May’. As for ‘easements’, we’ll come back to that unintentionally comical word lower down.
A year and a bit ago now, on 11 May 2020, the UK government put before parliament its plan for the way out of Covid-19 restrictions.
In the 50-page document, roadmap (spelled thus, as one word) appeared no less/fewer than seven times.
The Scottish devolved administration, however, spelled it as two words last year. This year the tartan politburo prefers the phrase ‘route map’. Anything to make on it is doing something different to the rest of the UK, yawn mega yawn.
But back to the matter in hand: between road map and roadmap, which is correct?
Well, obvs, both. If you look in the dictionaries – Oxford Online, Collins, Cobuild, Merriam–Webster, Webster’s New World College, Oxford Advanced Learner’s, Cambridge – you will find it typeset as two words. But here’s the thing. Words like this with a space between their two elements are what is known as ‘open compounds’ while roadmap and its ilk are ‘solid compounds’. Over time, almost like old couples, open compounds merge into solid ones.
It’s a historical process that has happened repeatedly. When Jane Austen wrote any body she did not mean ‘any old cadaver’; body in her sense meant ‘person’:
“It is very pretty,” said Mr. Woodhouse [sc. to Emma]. “So prettily done! Just as your drawings always are, my dear. I do not know any body who draws so well as you do.
When the great humanist Roger Ascham expressed his views on education, he wrote in deed, as was common, says the OED, until 1600 or thereabouts:
The Scholehouse should be in deede, as it is called by name, the house of playe and pleasure.
a1568 Scholemaster (1570) Pref.
Other examples going further back in time are alone (all adverb+one), although (all adverb+though) and albeit (all conjunction+be+it.) An analogous process produces the frowned-on *alot for a lot. The people who write it as a single word clearly perceive it as a unit of meaning in its own right and not dependent on the meaning of lot.
Someone on Twitter suggested that the road map spelling refers to the physical object and roadmap to the figurative meaning. It sounds vaguely plausible, but the only similar duo I can think of is black bird/blackbird, a distinction which is rather different.
In any case, the literal meaning is nowadays pretty rare (satnav rules, OK!) and searches in corpora show that each of the two forms is used for both literal and figurative meanings.
In an up-to-date corpus of 20 varieties of English, roadmap is about twice as common as road map. In a corpus built in 2104, the two forms were even-stevens, just about, but by the time of a 2018 corpus the ratio was 3:1 in favour of roadmap.
So, editors might be in a bit of a quandary if they come across road map. Should they change it to roadmap? The safest bet would be to raise it with the author and point out that the dictionaries are behind the curve on this one. I have it on good authority that the next Macmillan revision will change to roadmap. At the moment, Macmillan offers the solid form as an alternative to the open compound.
Meanwhile, let’s see how long we wait before most dictionaries mirror this new reality.
As for easements in Step 3 of the Road Map … it is probably just my scatological turn of mind that reminds me that the Tudor lard-arse Henry VIII built a magnificent Great House of Easement at Hampton Court capable of accommodating 28 of the lower orders for the relief of their bodily functions.
Oxford Online gives the word two meanings in current use, one being a legal right to cross or use someone’s land for a specified purpose, the other being a state of comfort and ease. Collins multiple offering says much the same while adding an architectural meaning. Merriam-Webster echoes Oxford.