You might well ask! Here’s some sample guidance::https://www.rospa.com/getattachment/8e57bd95-4173-4ae9-b31c-d92819f64a53/A-Guide-to-20mph-limits-revised-May23.pdfUnlike the national speed limits, 20mph is up to local authorities, which might explain the lack of consistency in their use, not helped by the confusing option of ‘zones’ or ‘limits’ or a combination of the two. While most guidance discourages the ‘blanket’ application of 20mph across a wide area and suggests specific traffic/safety reasons that should apply, many London Boroughs seem to have applied blanket limits/zones for broader, ‘make the world a better place’ reasons. One such is Ealing, where, despite the guidelines, they’ve applied 20mph to every road in the Borough, including lengths of dual carriageway. Others nearby boroughs haven’t done this, so you get the limit changing part way along the same road in an apparently arbitrary fashion as, for example, you pass between Ealing and Brent/Hounslow/Hillingdon, or from a TFL road to a local road. In addition, most official guidance suggests that, to be effective, a 20 limit should be applied to roads where drivers might reasonably expect there to be one, maybe because it’s naturally constricted or by a school, or suitable traffic calming measures have been installed. Yet in many places this is far from the case.
Peter Evans ● 30d