Nine criteria are discussed: they can be used to assess whether the effects attributed to road safety treatments in studies evaluating the effects of such treatments are causal or merely reflect randomness, confounding, or noncausal statistical association. The criteria are to a major extent based on similar criteria developed in epidemiology to assess the causality of factors associated with diseases. The criteria are related to (a) the presence of a statistical association between a treatment and changes in road safety (effect), (b) the strength of the statistical association between treatment and effect, (c) the internal consistency of the statistical association between treatment and effect, (d) the clarity of the causal direction between treatment and effect, (e) the invariance of the statistical association between treatment and effect regarding confounding factors, (f) the identification and measurement of causal mechanisms generating the association between treatment and effect, (g) the possibility of giving a theoretical explanation of the association between treatment and effect, (h) the presence of a dose-response pattern in the association between treatment and effect, and (i) the specificity of the association between treatment and effect to the intended target group of the treatment. These criteria can be used to assess the quality of road safety evaluation studies. The more criteria a study satisfies, the better is its quality. Use of the criteria is illustrated by means of examples.