Reducing traffic with “carrots”: A review of the evidence

Reducing traffic volumes is one way to reduce carbon emissions from the transport sector. Since increasing driving costs is often met with public resistance, high hopes are often pinned on the possibility to reduce traffic volumes by non-coercive policy measures, or “carrots”. Such measures include improvements of alternative modes, strategies that affect urban forms, and “soft measures” that aim to affect behaviour by providing information or changing norms and attitudes. This paper reviews the empirical evidence regarding such measures, focusing on their potential to reduce aggregate road traffic volumes in a national perspective. While such measures can yield significant other benefits, and may also reduce traffic volumes locally, our general conclusion is that their effects on aggregate traffic volumes appear small, especially from a climate policy perspective where emissions need to be cut radically and rapidly. While they are often motivated for several other reasons, overestimating their effects on aggregate traffic volumes may cause complacency, misallocations of scarce public resources and backlashes against climate policy.