There have been a number of attempts to estimate the level of UK coverage, of varying levels of sophistication, but I've not seen any that compare the length of roads mapped against actual road lengths.
The Department for Transport publishes statistics on actual road lengths by local authority here http://www.dft.gov.uk/pgr/statistics/datatablespublications/roadstraffic/roa dlengths/. The number of complete administrative boundaries plotted on OSM has shot up in the last few weeks, so it's now possible to compare actual road lengths (or at least DfT statistics) against the lengths of road that are in the map. Over the last couple of weeks, I've had a first attempt at doing this for about 100 local authorities with decent boundaries. Of the authorities I have managed to measure, the following all show more road mapped than the DfT believes exists: London Borough of Kingston upon Thames 111% Birmingham City Council 109% Rutland County Council 108% London Borough of Richmond upon Thames 107% Portsmouth City Council 106% City of London Corporation 105% London Borough of Waltham Forest 105% London Borough of Merton 105% London Borough of Redbridge 104% Solihull Metropolitan Borough Council 104% Reading Borough Council 104% London Borough of Hounslow 103% Kingston-upon-Hull City Council 103% London Borough of Sutton 103% Isle of Wight Council 103% London Borough of Barnet 102% London Borough of Islington 102% London Borough of Enfield 102% Southend-on-Sea Borough Council 102% London Borough of Brent 100% London Borough of Haringey 100% Given the scope for error in all this, the figures don't look too silly, and at least they suggests a pretty high level of coverage in these places. Eyeballing the map tends to confirm this. My measurements on the following authorities show the map holding less than half the roads that DfT believes exist: Ceredigion County Council 49% Gateshead Metropolitan Borough Council 48% Blackburn with Darwen Borough Council 45% Stoke-on-Trent City Council 45% Middlesbrough Borough Council 44% Rochdale Metropolitan Borough Council 42% Cornwall County Council 42% Bolton Metropolitan Borough Council 41% Sunderland City Council 41% Borough of Telford & Wrekin 41% South Tyneside Council 40% Barnsley Metropolitan Borough Council 40% Darlington Borough Council 34% North East Lincolnshire Council 33% Again, there is considerable scope for error, but these at least suggest a low level of coverage in these places. For anyone interested in the technicalities, I am doing this by loading a Planet OSM extract into a Postgis database. There are about 100 authorities where I haven't yet managed to extract a useable boundary, and a number of Counties where the ceremonial boundary that is plotted doesn't match the administrative boundary used by DfT. I hope this proves useful - if only so to suggest where to holiday in order to make the biggest impact. Hint: Cornwall, Cumbria, Norfolk and N. Yorkshire all seem to have a lot of un-mapped roads.
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