Variation in Speed Distributions with Local Truck Proportion in Mixed Freeway Traffic: A Shapley Decomposition

Variations in the overall speed distribution of mixed freeway traffic may reflect changes in vehicle-type weights, vehicle-type-specific speed distributions, or both. Using the highD vehicle trajectory dataset, we analyzed 50-m × 30-s spatiotemporal cells, with 60-s cells used for sensitivity analysis. Fixed-effects spline models estimated associations between local truck proportion (TP) and speed-distribution measures while adjusting for local traffic flow rate and recording–direction–location fixed effects. Standardized mixture distributions and a two-factor Shapley decomposition separated changes in the overall distribution into vehicle-type weight and within-type components. For the adjusted TP = 0.25 versus TP = 0.15 contrast, the differences in mean speed, the 25th percentile speed (P25), and the interquartile range were −2.51, −6.64, and +5.50 km/h, respectively, with the widening concentrated mainly on the lower-speed side. The directions of change were consistent at the 60-s scale. For mean speed, P25, the 50th percentile speed (P50), and the interquartile range, the vehicle-type weight component exceeded the within-type component in absolute magnitude at both temporal scales, as supported by recording–direction cluster bootstrap results. Distinguishing these components clarifies the statistical sources underlying changes in the overall speed distribution.