Random-Parameter Multivariate Negative Binomial Regression for Modeling Impacts of Contributing Factors on the Crash Frequency by Crash Types

Table 2

Summary statistics of independent variables.

Variable names	Definition	Min.	Max.	Mean	SD
Traffic characteristics
Interchange	1, interchange in the section (25.8%); 0, otherwise (74.2%)	0.00	1.00	0.25	0.44
Black spots	1, the crash number is higher than 18 (7.0%); 0, otherwise (93.0%)	0.00	1.00	0.07	0.26
AADT	Average annual daily traffic volume	31158	68836	46685.94	11969.59
Speed characteristics
	Operating speed of cars	88.92	134.37	121.91	21.70
	Speed difference of cars with the adjacent segment	−85.93	78.03	0.00	34.07
	Operating speed of trucks	60.38	104.86	78.80	12.34
	Speed difference of trucks with the adjacent segment	−34.01	25.72	0.01	19.89
	Speed difference between cars and trucks	12.64	73.99	43.11	16.99
Geometric characteristics
	Radius of the plane curve of the front section	5597.0	1000000.0	370713.6	478894.40
	Length of the plane curve of the front section	45.0	3677.0	1218.48	703.48
	Radius of the present plane curve	5597.0	1000000.0	370717.10	478891.70
	Length of the present plane curve	45.0	3677.0	1218.48	703.48
	Radius of the plane curve of the back section	5597.0	1000000.0	373043.70	479536.80
	Length of the plane curve of the back section	45.0	3677.0	1218.48	703.48
	Minimum longitudinal grade of the current section	−1.63	1.63	0.00	0.47
	Length of the longitudinal slope corresponding to the minimum grade	240.00	1740.00	773.33	296.03
	Maximum longitudinal grade of the current section	−2.50	2.50	0.00	0.97
	Length of the longitudinal slope corresponding to the maximum grade	362.00	1740.00	652.63	248.00
Sight characteristics
	Stopping sight distance of cars	244.00	1009.90	439.06	142.23
	Stopping sight distance of trucks	52.00	279.50	87.48	29.92
	Transverse clear distance of cars	0.02	6.59	1.66	1.71
	Transverse clear distance of trucks	0.00	0.47	0.06	0.06