Making words work better
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. 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'.
Modern usage follows Shakespeare. In the GloWbE corpus (Global Web-based English), in the meantime is 20 times more frequent than in the meanwhile.
Two poetic "meanwhiles"
And, as I was writing this, the last words of Auden’s Friday’s Child, in memory of the German theologian Dietrich Bonhoeffer, floated into my head:
The rule seems to be that if a candidate can recite half a dozen policy positions by rote and name some foreign nations and leaders, one shouldn't point out that he sure seems a few whereases shy of an executive order.
The above is a superlative example of the creative potential of the idiom frame ‘a few X short/shy of a Y’, e.g. ‘a few fries short of a Happy Meal’.
As a further historical footnote, it is interesting that the legalistic, ritual use of whereas as a preamble to legal documents led to its being used as a noun, defined as follows in the Urban Dictionary of its day, Grose’s 1796 Classical Dictionary of the Vulgar Tongue:
Jeremy Butterfield Editorial 