FOI release

'A' roads: reference speeds used to calculate delays to journeys

Freedom of Information response about the reason we set lower 'A' road reference speeds on dual carriageways than on single carriageways.

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This Freedom of information response concerns the reasons we set a lower speed expectation for dual carriageway ‘A’ roads compared with single carriageway ‘A’ roads with regard to the inter-urban congestion statistics for January 2011. The reference speeds queried were used to calculate delays to journeys made on the strategic road network (SRN).

Our response explains that reference speeds are calculated from survey data and that the difference in speeds for single and dual carriageways likely come from the specific characteristics of these types of road on the SRN. Factors affecting reference speeds are:

  • traffic density during off-peak periods
  • road surface conditions
  • the presence of traffic calming measures such as speed cameras
  • the exact layout of the road and location of junctions such as roundabouts

Speed surveys have now been replaced and traffic speeds on the SRN are now supplied from the Highways Agency’s traffic and information system.