Well, yes, you've got it in one and expressed it very well. The whole point of a DPF is to gather particulates on start-up, then burn them off at high temperature later in a car's journey. If the car doesn’t go anywhere this doesn’t happen. So engineers have developed faster regenerating DPFs that contain heating elements and extra fuel is injected directly into the DPF itself rather than the engine combustion chambers. Others (such as Mercedes and PSA/Ford) use additive-fuelled DPFs, You are right that this is a case where legislation has forced technology onto manufacturers without sufficient time to develop it properly, so that task has befallen the poor old consumer. It's why manufacturers are now favouring small capacity turbocharged petrol or supercharged petrol engines to diesels.