where y is the fitted freight volume, x1 is GDP, x2 is the fixed investment.
Fitted freight volume versus actual freight volume based on
MLR, NLR, and SLR is shown in Fig. 2. But in [7], the predicted
transport data is compared with actual data. Finally the polynomial
model gives the best result. The model based on NLR is the polynomial
model in this paper. So we can get the root mean squared error of
calibration (RMSEC) of MLR, NLR, and SLR models from T