Understanding Motorcycle Emissions Across Their Technical, Behavioral, and Socioeconomic Determinants in the City of Kigali: A Non-Parametric Multivariate Analysis

Motorcycle emissions are a growing environmental and public health concern in many low- and middle-income countries. While several studies have examined the emission profiles from larger vehicles in urban areas, very few have analyzed motorcycle emissions through a parametric and non-parametric multivariate lens, combining technical, behavioral, and socioeconomic factors, a gap that this study attempts to address. MANOVA and Kruskal–Wallis H test analyses highlighted visible smoke emissions (hydrocarbon: H = 30.62, p < 0.001; carbon monoxide: H = 16.71, p < 0.001; dioxygen: H = 6.67, p = 0.010), year of manufacturing (carbon monoxide: H = 20.61, p < 0.001; hydrocarbon: H = 11.80, p = 0.008), average fuel consumption (carbon dioxide: H = 13.32, p = 0.004), and daily driving distance (carbon monoxide: H = 10.09, p = 0.018) as significant predictors of emissions. The results also indicate that newer and well-maintained motorcycles (2018–2021) consistently showed the lowest carbon monoxide and HC levels compared to the older and poorly maintained counterparts with the highest emissions. Consistently, motorcycles with visible smoke showed substantially elevated carbon monoxide and hydrocarbons and reduced dioxygen, establishing visible smoke as a practical marker for excessive emissions. Additionally, younger riders (19–28 years) exhibited higher hydrocarbon emissions, while greater riding experience and more passengers influenced dioxygen levels. Spearman correlation analysis reinforced these patterns, with visible smoke showing strong positive correlations with carbon monoxide (ρ = 0.21) and hydrocarbon (ρ = 0.28), and carbon monoxide was negatively associated with motorcycle age (ρ = −0.18). These findings underscore manufacturing year, vehicle maintenance, and visible smoke as practical, high-impact targets for reducing motorcycle emissions, offering a basis for targeted emission control strategies.