Induced traffic

An excellent report on the impact of road investments has been commissioned by the Campaign to Protect Rural England from Lynn Sloman and colleagues. This report re-examines the outcomes of a number of Highways England road schemes, finding average increased traffic above control levels in the short run (3-7 years) of 7% and in the longer run (8-20 years) of 47%. This is clear evidence for substantial ‘induced traffic’ generated by new capacity. The CPRE study also find very limited evidence for benefits to the local economy from road investments.

These findings pose problems for the  Department for Transport’s WebTAG approach to the economic appraisal of road schemes, in which time savings to users is the dominant benefit. However, induced traffic tends to restore congestion to what it had been, lessening time savings. If induced traffic were properly taken into account, the apparent value of the investment would be substantially less. Moreover, the orthodox approach assumes unchanged land use, yet the CPRE report documents extensive land use change in four detailed case studies. Such changes have implications for local economies, not necessarily wholly advantageous if these take the form of low-wage employment in warehouses or car-dependent residential development.


The main policy objective of the current UK national road investment programme is to boost economic growth by improving inter-urban connectivity and reducing congestion. Each scheme of the programme must offer acceptable value for money on the orthodox approach to appraisal, yet this approach overstates benefits by underestimating induced traffic and disregards changes in land use made possible by improved access. We need an evidence-based approach to investment appraisal, in which careful evaluation of completed schemes of the kind commissioned by the CPRE informs appraisal of prospective investments.