This review reflects the difficulties of applying a clinical trials paradigm to studies evaluating the impact of the environment on physical activity. It also shows that relationships
between objectively measurable environmental modifications and cycling have been less thoroughly examined than with walking or physical activity in general. In the UK, there have been calls to consider environmental changes, such as cycle route construction, as opportunities for good quality research.24 Future research should consider economic evaluations,
measures of population benefit and equity assessment, to examine whether built environment interventions impact on all population groups equally. However, as this review indicates, such interventions may be subject to an ‘inverse evidence effect’, whereby interventions having potential to influence population health the most are those in which controlled trials are least practicable.55 The challenge for future research is to bridge this evidence gap with innovative research designs, and to avoid publication bias where possible.