Effects of weather conditions, light conditions, and road lighting on vehicle speed

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Table 4 Results of multivariate adaptive regression splines (MARS) analysis showing model, dependent variable, type

Model		Results
Weather conditions and dependent variable	GCV	RSS	RSq
Clear
Average speed	16.8	2,734,006	0.97
Speed difference	16.8	2,734,006	0.71
Rain
Average speed	20.2	549,327	0.97
Speed difference	20.2	549,327	0.66
Snow
Average speed	31.1	632,439	0.92
Speed difference	31.1	632,439	0.59

GCV generalized cross validation, RSS residual sum-of-squares (RSS) of the model, and Rsq R-squared of the model. For more information see Milborrow (2015). Model: dependent variable ~ natural light condition (day, darkness, twilight) + distance to the nearest intersection + road width + posted speed limit + road surface temperature + year + month + presence of road lighting or not + amount of precipitation + weather condition (clear, rain, snow) when applicable. Model with the best fit shown in italics