Evaluating the impact of zone 30 policies on citywide road traffic emissions and efficiency: A big data-driven approach

Zone 30, i.e., decreasing road speed limits to 30 km/h, has gained popularity in Europe recently. While its safety benefits are well documented, its impacts on emissions and efficiency remain less clear. This study uses large-scale trajectory data from over 3.4 million trips in Milan, Italy, to assess Zone 30’s impacts on road traffic emissions and efficiency across different scenarios. Results show that in the most restrictive scenario, where Zone 30 applies to all roads except highways and primary roads within the city’s outer ring, the average travel time increases by 7.24%, and total emissions rise by 0.66% (PM non-exhaust), 1.93% (CO), 2.06% (CO2), 2.12% (NOx), and 3.53% (PM exhaust). These impacts vary across space and time: peak hours and cross-region trips show the largest increases, while roads in city centers with lower functional classes and higher centrality experience smaller relative increases. As Zone 30 initiatives are usually accompanied by efforts to curb driving demand, further analysis suggests that a minimum 5% reduction in total vehicle travel would offset the emission increases and achieve an overall breakeven. Together, these findings offer policymakers a data-driven understanding of Zone 30’s impacts and provide a flexible, scenario-based framework to guide nuanced, phased implementation strategies.