Construction logistics (CL) is assumed to represent 20-35% of total urban freight traffic and account for a sig- nificant share of environmental nuisances. However, methodologies used so far make abstraction of travelled vehicle-kilometres (vkm), hence inadequately determining the true environmental impact of off-site CL activities. In turn, the lack of baseline assessments renders the development of sector-specific transport policies difficult. In Belgium, the use of On-Board Units shows promising results to answer this research gap. This paper presents a methodological approach to derive CL vkm on vehicle and trip level, based on algorithmic (R) and geospatial (GIS) analyses of GPS data from all HGV driving in or through the territory of Belgium, which serves as input to conduct a city-wide environmental impact assessment in terms of external costs. The proposed methodology was deployed on 66 large construction sites in the Brussels-Capital Region (BCR) active between 2020-2022, during the month of September 2021. Subsequently, results were translated to monetary terms to capture the generated environmental and mobility impacts. With its 968,041.96 monthly driven vkm, CL represents 26.40% of total HGV traffic in the BCR. This share generates €45,631.85 of external costs per workday, totaling €1,003,900.61 per month. Particular attention is paid to local air pollution (NOx, PM) and global emitted pollutants (GHG; CO2- eq.) which account for €55,123.07 and €80,409.95 per month of damage costs, respectively. To mitigate these damage costs and meet environmental goals, governments should pay increasing attention to urban construction transport by stimulating CL setups or developing emission-free public procurement procedures. The results of this study can serve as baseline for future policy recommendations and scenario evaluations.