Model Category of independent variables Model equation Coefficient significance R² Linear type Visible distance ( ${x}_{1}$ ) $y=-0.001{x}_{1}-0.001{x}_{2}-0.139{x}_{3}+15.440$ X 0.637 Traffic volume ( ${x}_{2}$ ) O Speed ( ${x}_{3}$ ) X Constant O Visible distance ( ${x}_{1}$ ) $y=-0.003{x}_{1}-0.001{x}_{2}+4.420$ O 0.570 Traffic volume ( ${x}_{2}$ ) O Constant O Visible distance ( ${x}_{1}$ ) $y=-0.001{x}_{1}-0.153{x}_{3}+15.956$ X 0.552 Speed ( ${x}_{3}$ ) X Constant O Visible distance ( ${x}_{1}$ ) $y=-0.002{x}_{1}+3.760$ O 0.471 Constant O Exponential type Visible distance ( ${x}_{1}$ ) $y={\text{e}}^{-0.001{x}_{1}-0.0003{x}_{2}-0.052{x}_{3}+5.688}$ X 0.681 Traffic volume ( ${x}_{2}$ ) O Speed ( ${x}_{3}$ ) X Constant O Visible distance ( ${x}_{1}$ ) $y={\text{e}}^{-0.001{x}_{1}-0.0004{x}_{2}+1.567}$ O 0.630 Traffic volume ( ${x}_{2}$ ) O Constant O Visible distance ( ${x}_{1}$ ) $y={\text{e}}^{-0.001{x}_{1}-0.058{x}_{3}+5.910}$ X 0.594 Speed ( ${x}_{3}$ ) X Constant O Visible distance ( ${x}_{1}$ ) $y={\text{e}}^{-0.001{x}_{1}+1.286}$ O 0.530 Constant O Natural log type Visible distance ( ${x}_{1}$ ) $y=\text{ln}\left(-0.022{x}_{1}-0.029{x}_{2}-5.272{x}_{3}+517.81\right)$ X 0.419 Traffic volume ( ${x}_{2}$ ) X Speed ( ${x}_{3}$ ) X Constant X Visible distance ( ${x}_{1}$ ) $y=\text{ln}\left(-0.072{x}_{1}-0.031{x}_{2}+99.566\right)$ O 0.353 Traffic volume ( ${x}_{2}$ ) X Constant O Visible distance ( ${x}_{1}$ ) $y=\mathrm{ln}\left(-0.017{x}_{1}-5.771{x}_{3}+536.003\right)$ X 0.346 Speed ( ${x}_{3}$ ) X Constant X Visible distance ( ${x}_{1}$ ) $y=\mathrm{ln}\left(-0.071{x}_{1}+76.136\right)$ O 0.266