I've heard back from the Department for Transport on how they measure motorway slip roads for their road length statistics, and it seems that they don't.
Assuming that "motorway link" in OSM roughly corresponds to the slip roads that DfT are omitting, that would explain why the length of motorways that I am measuring from the OSM database is higher than the figures the DfT publish. They also tell me that they run an annual consultancy to agree road lengths for each local authority. In July they generate an estimated total road length (for each road classification) for Local Authorities using their own major Roads database for Motorways and A roads, the Ordnance Survey's Integrated Transport Network for minor roads, and totals that they have already agreed with Local Authorities. The estimates they generate are then validated by Local Authorities against their own road management systems and any differences highlighted. DfT then evaluate any differences that the local authorities flag up, and adjust the totals accordingly. Previous annual updates have been published in November, so presumably we can expect the revised figures in a few months time. They also tell me that they are adapting their system to take account of slip roads, but I am not clear when we will see the results of this. In terms of the overall coverage figure that I am generating for each local authority this makes very little difference - motorways are a small part of the total, and link-roads are only a proportion of that. The rough calculations that I have done indicate that if I excluded slip roads the the OSM figure for "real" motorways would be remarkably close to the DfT figure. Next time I extract and process the full data set I will make the appropriate adjustment so we can see the results for each local authority. I'm intending to pull separate figures for the motorway itself, and for the slip roads, so there is a basis for comparing both the equivalent DfT figure and the complete content of the OSM database.
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