"We agree that many of the interventions about which we found evidence were unlikely to have large effects in the face of strong economic and personal interests to the contrary. The complexity of transport related behaviour and interventions also makes it difficult to establish causal relations between interventions and outcomes. However, these do not constitute arguments against our approach. The evidence about barriers to cycling is undoubtedly relevant. But measures that seem to address these factors cannot be assumed to lead necessarily to the desired results. The UK Department for Transport recently admitted that the National Cycling Strategy has had no effect on the overall level of cycling in England despite its good intentions. One reason is a lack of political will to address the deeper causes and competing interests, another is that transport policy often seems to pursue conflicting goals. We hinted in our discussion that more ambitious measures might be more effective, and our findings are consistent with (if not proof for) a view that much more radical changes in society would be required to achieve significant population health gain through a modal shift towards walking and cycling. Carnall complains that we treated driving as a disease and ignored the social forces which underlie it, but we set out with an entirely open mind about what types of "intervention" might be relevant, searching for evidence accordingly (see bmj.com). We hoped to find evidence about the effects of policies on, for example, car advertising and suburban planning and would certainly have included it had we found it. Readers may have been unduly influenced by the editorial summary for This week in the BMJ. This contains an error of fact in the second sentence—"Reviewing 22 studies analysing the effect of targeted behaviour change programmes"—and readers will struggle to find the opening sentence—"Encouraging people to use alternative and healthier modes of transport may not be enough to improve the health of the population"—anywhere in our paper. We did not see the paragraph before publication and do not endorse it now." (Author/publisher)
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