An Investigation of the Safety Performance of Roundabouts in Korea Based on a Random Parameters Count Model

Table 5

Estimation results of roundabout accident frequencies in Korea.

Variable	Constant Parameter		Random Parameter
	Mean	t-statistic	Distribution	Mean	t-statistic	Standard Deviation	t-statistic
Constant	-1.82	-1.41	n.a
Exposure
Logarithm of AADT	0.23	2.51	n.a
Geometrics
Average width of entry lane (m)	0.16	1.96	n.a
Presence of Central island	1.00	2.29	n.a
Width of circle lanes(m)	-0.29	-1.93	n.a
Radius of entry lanes(m)	n.a		normal	-0.04	-4.94	0.02	5.13
number of entry lanes	n.a		normal	0.14	3.34	0.07	4.4
Presence of Pedestrian crosswalk	n.a		normal	0.65	2.61	0.17	1.98
Vertical grade	n.a		normal	-0.63	-3.49	0.62	4.26
Scale Parameter for overdispersion	1.58	4.7
Number of observations				199
Log likelihood at convergence of RPNB model				-382.82
Log likelihood with constant only				-1,123.89

n.a: not applicable.