Question to the Department for Transport:

To ask the Secretary of State for Transport, what definition her Department applies to determine when a road network is operating (a) at and (b) over full capacity.

Transport Business Cases follow the HM Treasury five-case Business Case model. Decisions are informed by the assessment of the scheme in relation to strategic fit, value for money, deliverability, commercial and financial considerations. The Department provides guidance to scheme promoters in DfT’s Transport Analysis Guidance (TAG).

The relationship between levels of demand and road capacity would be considered as part of the transport modelling and analysis for potential transport investments.

An introduction to this modelling, including the interaction of supply conditions and road demand, is provided in Chapter 3 of the TAG guidance for the Senior Responsible Officer. Further technical detail is provided in Appendix E of TAG unit M3.1, TAG unit M3.1 highway assignment modelling.