A DPF filter is designed to collect soot at all vehicle speeds. It still operates in an urban environment with either a 20mph or 30mph limit, collecting soot and depositing it within the filter. However the filter relies upon a process of “passive regeneration” to periodically burn off the collected soot. This would normally happen if the vehicle is running at a sustained speed of 40mph or above. Hence current urban environments with a 30mph limit would not provide the opportunity for such “passive regeneration”. In such cases the engine is designed to automatically trigger a process called “active regeneration” at a pre-determined level of soot collection (normally 45%). In this extra fuel is injected into the engine automatically to raise the temperature in the filter and burn off the collected soot. This process burns off the soot already collected in the filter. This process is already used for vehicles operating in urban 30mph environments.
Hence a 20mph limit will not create any difference in the way that a Diesel Particulate Filter clears itself. A 20mph environment will be no more of a problem than a 30mph environment. If the vehicle is not periodically driven above 40mph then it will simply initiate its own "active regeneration".