Background: In the city of Maastricht in the Netherlands, a highway crossing several deprived neighborhoods wastunneled in 2016. The vacant space on top of this tunnel was redesigned and prioritized for pedestrians andcyclists. The aim of this study was to evaluate the effect of this major infrastructural change, named the GreenCarpet, on total and transport-based physical activity (PA) levels.Methods: Participants (≥18 years) were part of one of three area-based exposure groups. The maximal exposuregroup lived in neighborhoods directly bordering the Green Carpet. The minimal exposure group consisted ofindividuals living at the other side of the city, and the no exposure group consisted of individuals living in a nearbycity. Actual use of the new infrastructure was incorporated as a second measure of exposure. Data were collectedbefore and 3-15 months after the opening of the Green Carpet. Device-based measurements were conducted toobtain PA levels and collect location data. Changes in PA over time and intervention effects were determined usinglinear mixed models.Results: PA levels in the Green Carpet area increased for the maximal and minimal exposure groups, but did notlead to an increase in total or transport-based PA. For the no exposure group, transport-based MVPA decreased and transport-based SB increased. The significant interaction (time x exposure) for transport-based SB, indicated differences in trends between the no exposure and maximal exposure group (B=-3.59, 95% CI - 7.15; -0.02) and minimal exposure group (B= -4.02, 95% CI -7.85, -0.19). Trends in the results based on analyses focusing on actual use and non-use of the new infrastructure were similar to those of the area-based analyses.Conclusions: Results suggest that the Green Carpet led to more PA in this specific area, but did not increase the total volume of PA. The area-based differences might reflect the differences between users and non-users, but we should be careful when interpreting these results, due to possible interference of selective mobility bias. This paper reflects that the relationship between infrastructure and PA is not unambiguous.