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| Variable category | Symbol | Variables |
|
| Traffic flow characteristics | C | Correlation coefficient between occupancy and flow during 5–10 minutes |
| Cnt/Spd/Occ | Average 30 s vehicle count/speed/detector occupancy during 5–10 minutes (veh/30 s)/(mile/h)/(%) |
| Stdc/Stds/Stdo | Std. dev. of 30 s vehicle count/speed/detector occupancy during 5–10 minutes (veh/30 s)/(mile/h)/(%) |
| Cc/Cs/Co | Coefficient of variation of 30 s vehicle count/speed/detector occupancy during 5–10 minutes (veh/30 s)/(mile/h)/(%) |
| Dc/Ds/Do | The difference in average 30 s vehicle count/speed/detector occupancy between 15–20 min and 5–20 min (veh/30 s)/(mile/h)/(%) |
| Lcnt/Lspd/Locc | Average difference in 30 s vehicle count/speed/detector occupancy between adjacent lanes during 5–10 minutes (veh/30 s)/(mile/h)/(%) |
| LstdcLstds/Lstdo | Std. dev. of difference in 30 s vehicle count/speed/detector occupancy between adjacent lanes during 5–10 minutes (veh/30 s)/(mile/h)/(%) |
| Environmental characteristics | Vi | Visibility (mile) |
| We | 1 = bad weather condition; 0 = otherwise |
| Rs | 1 = otherwise; 0 = dry road surface |
| Li | 1 = bad light; 0 = otherwise |
| Geometric characteristics | Lw | Lane width (ft) |
| Is | Inner shoulder width (ft) |
| Os | Outer shoulder width (ft) |
| Ra | 1 = ramp segment; 0 = otherwise |
| Crash characteristics | Tr | 1 = the crash including a truck; 0 = otherwise |
| Se | 1 = the injury crash; 0 = otherwise |
| Ho | 1 = the hit object crash; 0 = otherwise |
| Ss | 1 = the sideswipe crash; 0 = otherwise |
| Re | 1 = the rear end crash; 0 = otherwise |
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