Urban infrastructure renewal significantly influences active mobility patterns, offering opportunities to enhance urban liveability and accessibility. Much of the existing research on active mobility has been cross-sectional and is susceptible to self-selection bias, where individuals who prefer active travel tend to live in walkable neighborhoods. To address this limitation, this study employs a longitudinal approach to analyze changes in pedestrian and cyclist behavior on Viale Argonne in Milan during and after an urban renewal project. Using Strava Metro data, the analysis reveals substantial increases in leisure pedestrian trips, particularly during evening hours and among younger (18–34) and middle-aged (35–54) demographics, highlighting temporal and demographic variations. All activity types exhibited a spatial shift from sidewalks to the redesigned raised median island, reflecting the enhanced streetscape’s appeal. While leisure mobility speeds generally decreased, commute mobility speeds showed variable increases. Gender differences in activity levels remained negligible. These findings underscore the value of longitudinal methods and objective mobility data in evaluating urban design interventions. The results provide actionable insights for sustainable urban design, emphasizing the role of inclusive and well-planned infrastructure in fostering active travel and improving public health outcomes.