3.3.1. Dependent Variables

The first group of the regression models used "mean vehicle speed" at a recording site as the dependent variable. Mean speed was the arithmetic average of the travel speeds of the individual vehicles in the recordings of the three extracted periods at each recording spot. Because there were just nine recording spots in total, the number of observations seemed too few to carry out stable multiple regression results. Therefore, we used the morning, afternoon, and evening recordings before (September 2014) and those after (June 2015) the PPS as separate samples. Thus, the first analysis had 54 observations (9 × 2 × 3). Gujarati argued that 40 observations would never be too small if a model specification is correctly done [60] (p. 484).

The second group of the regression models used "difference in the mean speed before and after the PPS" as the dependent variable. There were 27 observations (9 × 3), which did not meet the minimum sample size of 30 observations for regression analysis. However, the regression results were appropriate for verifying the causal relationships found in the first regression analysis.
