When I interviewed him, he said it was improvised.
That's really cool. Listening to it again yesterday, I considered that it may've been rehearsed/written so that each chord change had its own solo part and it was broken down that way and pieced back together but apparently not! It remains a stunning piece of work and one of my all-time favourite solos.

What's the betting Numan drops On Broadway from the set of his 30th anniversary tour of The Pleasure Principle? Mind you, he could reunite the old band and retain his current bassist...