IELTS Grammar: 'onto' or 'on to'?

In Thursday's lesson I wrote this: "Traffic will be diverted on to a dual carriageway".

But why didn't I write 'onto' as one word? Do you know when to write the single word 'onto' and when to write 'on to' as two separate words?

Here's the answer:

• When the meaning is 'to a position on the surface of', we can use the single word 'onto' e.g. The child climbed onto the chair.
• But if the meaning is 'onwards' or 'towards', we should use the two words 'on to'. In my example ("on to a dual carriageway"), the meaning was 'towards', not 'to a position on the surface of'.