Assessing the limitations of motorization rate as a proxy for local climate-related variables: Evidence from Bulgaria and Sofia

• Motorization rate evaluated as proxy for climate indicators. • Empirical analysis for Bulgaria and Sofia (2004–2022). • Weak association with temperature detected in regression analysis. • No statistical relationship found with precipitation. • Results highlight limits of stock-based transport indicators. This study evaluates whether the number of registered passenger cars (motorization rate) can be used as a standalone proxy indicator for local climate-related variables. Using official datasets for Bulgaria and its capital city, Sofia, over the period 2004–2022, the analysis examines the relationship between vehicle registration counts and two aggregated climate indicators: average annual air temperature and average annual precipitation. Correlation and regression analyses reveal weak and statistically non-significant associations with temperature, along with negligible explanatory power for precipitation. These findings indicate that vehicle registration counts alone are insufficient for explaining or monitoring variation in aggregated climate-related variables. Building on this negative result, the study outlines a methodological framework for future transport–climate research based on activity- and composition-oriented indicators, including vehicle-kilometres travelled, fleet structure, and higher temporal resolution data.