A new CDV study shows that the effectiveness of traffic calming measures may decrease over time as drivers gradually become accustomed to them and their impact on speed reduction weakens.
Abstract
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.
Citation
Jiří Ambros, Martin Šípek, How does the novelty effect influence speed impacts? An exploratory meta-analysis of five types of traffic calming measures, Transportation Research Interdisciplinary Perspectives, Volume 37, 2026, 102037, ISSN 2590-1982, https://doi.org/10.1016/j.trip.2026.102037