Threshold Effects of Motorway Density on Road Accidents in Poland

Abstract

Using annual national-level data for Poland (2010–2024), this study applies Hansen’s threshold regression framework to identify the point at which the historical positive relationship between motorisation and road traffic accidents reversed sign. A statistically significant and robust structural break is detected in 2019–2020 when the density of motorways and expressways exceeded approximately 1.36 km per 100 km² of territory (95% CI: 1.31–1.46). In the pre-threshold regime (2010–2019), a 10% increase in passenger cars per 1000 inhabitants was associated with an 11.2% rise in accidents; after crossing the threshold (2020–2024), the elasticity became – 0.94. The elasticity of high-speed road infrastructure switched from statistically insignificant to significantly negative (−0.41). Robustness checks using fatalities, injuries, alternative threshold variables, and a two-threshold specification confirm the results. The findings provide the first formal evidence of a system-level safety threshold in Central Europe and demonstrate that sufficiently dense grade-separated infrastructure can trigger a genuine “safety-in-numbers” effect at the national level. Policy implications are drawn for Romania, Bulgaria, and Western Balkan countries still below the identified threshold.