One or Two?

I was writing a paper when I needed to use the word rollercoaster. Spell check corrected my "misspelling" and told me it was roller coaster. Two words. This interested me, so I searched the spelling of rollercoaster or roller coaster online. Their seemed to be about the same amount of people who thought rollercoaster was spelled with one word than people who thought roller coaster was spelled with two.

So, my question is, is rollercoaster/roller coaster spelled with one word or two?