How does the novelty effect influence speed impacts? An exploratory meta-analysis of five types of traffic calming measures

• Novelty effects reduce speed impacts of traffic calming measures (TCMs) over time. • Both physical and non-physical TCMs show temporal declines in effectiveness. • Meta-analysis quantifies short- and longer-term speed effects across five TCM types. • Short-term evaluations tend to overestimate long-term TCM impacts. Although traffic calming measures (TCMs) have been used internationally for several decades, their reported speed impacts remain inconsistent, partly due to differences in evaluation design and follow-up duration. One underexplored source of bias is the novelty effect, whereby the initial effectiveness of an intervention may decline over time as drivers become familiar with it. This exploratory study examines the influence of novelty effects on speed outcomes for five common traffic calming measures (3D pedestrian crossings, driver feedback signs, optical speed bars, reduced speed limits without enforcement, and physical vertical measures). Based on 51 international studies and 275 effect estimates, a random-effects meta -analysis with time-based subgroups was conducted. The results indicate that effectiveness generally decreases over time across all intervention types. After adjusting for publication bias, the decreases were approximately between 40% and 100%, which means that the effectiveness of TCMs may be halved or even nullified over time. The findings suggest that no category of TCMs is immune to novelty effects and that short-term evaluations are likely to overestimate long-term effectiveness. These results have important implications for traffic calming evaluation, programme design, and evidence-informed transport policy.