No, it does not at all require the car to travel with constant speed. But a higher engine load in general is beneficiary, since then the exhaust from the engine into the DPF is warmer, and that improves the soot incineration.
Going slowly, stopping with the engine idling - that's the worst condition.
But going up and down in speed, from midrange to high, is no problem at all. On the contrary, it probably involves higher load during acceleration, and that accelerates (!) the cleaning process.
Towing a caravan improves filter cleaning quite a lot. But it also fills the filter faster, so there's probably no net gain.
If the engine coolant hasn't reached normal temperature, around 88°C, the cleaning process doesn't start at all.
In cars where you can't see in the display that regeneration is active, the most obvious sign is that consumption doesn't display 0.0 l/100 km when you are coasting.
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