Guidance for transport impact evaluations: choosing an evaluation approach to achieve better attribution

Summary

One of the main challenges in undertaking impact evaluations of transport interventions is the ability to demonstrate that the observed outcomes and impacts have been caused by the intervention, confidently ruling out the influence of external factors.  This guidance has primarily been designed to focus on the approaches which can attribute the outcomes and impacts to the intervention and provide some considerations to take when undertaking these evaluation approaches within a transport context.

Due to the specific circumstances underlying each evaluation, it is recognised that there are many varied, and sometimes conflicting, aspects to consider when designing an impact evaluation and whilst the ability to attribute the findings back to the intervention is considered to be very important, it might not be the central requirement for some evaluations.  Therefore, the guidance has been designed to provide a systematic process for considering the most suitable evaluation approach which is tailored to the type of intervention being delivered and the evidence requirements.  However, due to the specific circumstances underlying each evaluation, this guidance is not intended to be prescriptive but provide a flexible framework for selecting the overarching evaluation approach.

As the primary objective of the guidance is to consider impact evaluation approaches which enable the attribution of the outcomes and impacts to the intervention, this document does not constitute comprehensive guidance on all aspects of transport evaluation (for example undertaking a process evaluation), neither does it provide detailed guidance on how to evaluate specific types of interventions.  However, the earlier steps in the evaluation design will be relevant for all types of evaluation.

This step-by-step guidance has been developed by the Tavistock Institute and AECOM to support policy makers, delivery bodies and analysts from the point at which they have decided to undertake a transport impact evaluation.