Our resident cops can no doubt provide a definitive answer, but I *think* he only goes to jail if he commits a crime, rather than a driving offence - I don't think speeding would do it (the law distinguishes between offences and crimes).
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The phrase often used is 'the suspended part of sentence may be activated if you commit another offence punishable by imprisonment'.
Thus another dangerous driving would do it, but a careless would not.
As CopperCyclist says, activating the suspended sentence is by no means automatic.
The courts tend to take a dimmer view if it's like offending.
Thus someone on a suspended sentence for violence might stay out if his next offence is, say, minor drugs possession.
But if he fills someone in again, he will likely go inside, often by recall before the courts get their hands on him.