Roadspace Reallocation Schemes Should Take Account of Traffic Evaporation, Recommends Report

A report about the benefits of active travel schemes recommends that schemes which reallocate roadspace away from general traffic should be appraised differently than they are now.

Instead of assuming that the same amount of traffic will have to squeeze into a smaller space or take a different route, guidance and tools should take account of the reality that often in response to capacity reduction, motor traffic reduces (traffic evaporation).

The report is by John Nellthorp of the Institute for Transport Studies at Leeds University, and was commissioned by Active Travel England. It was finished in December 2023, but only published by the government last month.

The report says:

‘The issue in appraisal is that roadspace reallocation projects reduce network capacity by some modes – particularly by car – in order to increase it for other modes.

It then becomes absolutely essential to capture the benefits of the change and not to overestimate the disbenefits of the change’.

This means taking account of behavioural adaptations that will occur due to the scheme, including change of route, change of departure time, change of mode, trip frequency etc.

‘These individual behavioural adaptations combine to reduce the pressure on the road whose capacity has been reduced – the impact is shared out across the network, over time and across modes.

If the transport authority provides alternative forms of capacity – e.g. cycleways or better public transport – there may well be an increase in accessibility overall’.

Some models only take account of change of route, and therefore show increased congestion on other parts of the network.

An academic study by Cairns, Hass-Klau and Goodwin shows that as well as traffic reduction on the route itself, capacity reduction leads to an average traffic reduction of 25% in the area as a whole.

Transport Appraisal Guidance (TAG) and the Active Modes Appraisal Tool (AMAT) do not address how to forecast the impacts of road capacity reductions

The report says that current guidance is suitable for active mode investments that don’t affect mixed traffic capacity – for example a new off-road cycle track.

For schemes that affect traffic capacity, e.g. a cycle track that takes a general traffic lane, they ought to take account of these facts:

• there is likely to be some traffic evaporation
• the extent of this is variable, depending on the alternatives available to people
• a model which assumes that volumes of car trips stay the same but shift route is likely to produce unrealistic results and overstate any disbenefits

Currently councils use tools which show that roadspace reallocation results in large disbenefits due to traffic delays which are ‘an artefact of the model, not the reality’.

The report recommends that Active Travel England and the DfT look at the available evidence then issue suitable guidance.

Other Aspects of Active Travel Appraisal

The report doesn’t only consider roadspace reallocation schemes, it looks more widely at how economic appraisal of active travel interventions could be done better.

It finds that current guidance and tools, such as AMAT, is quite good but with notable gaps.

Among the gaps are the way appraisal treats active travel in relation to quality of life benefits, wider economic impacts such as reduced workplace absence, travel time savings for walking and cycling, and safety benefits.

The report says that there is evidence to demonstrate that active travel has health/quality of life benefits, but there is not yet an accepted systematic method for quantifying them in response to active travel interventions.

Similarly, AMAT does not yet include time savings on walking and cycling trips as benefits.

The report also argues that local authorities with an area-wide strategy should be able to forecast levels of walking and cycling based on the range of measures they are taking, rather than only in relation to the specific link that their current scheme focuses on.

Taking account of the area-wide strategy would mean higher rates of active travel were forecast, and result in a greater Benefit-Cost Ratio.

Related to this, there is an argument for undertaking most active mode appraisal at strategy level, and doing ‘light touch’ appraisal for an individual scheme that forms part of the strategy.

The report says that health cost savings from active travel are not currently accounted for in BCR calculations, but should be.