Table 3
Negative Binomial Regressions for Number of Person-Trips: All Purposes and Recreational Purposes
Variable
Beta
SE
t-stat
P-value
Negative binomial regression
Poisson regression
Dependent variable: number of person-trips for all purposes
Constant
1.672
0.012
138.00
0.000
HH members per vehicle owned
0.255
0.004
59.20
0.000
Income per household member
�16E-06
2.77E-07
�.40
0.000
Population density
�42E-06
6.10E-07
�25
0.000
Vehicle age
�014
0.001
�.80
0.000
Weekend day indicator
0.100
0.007
13.50
0.000
SUV indicator
0.045
0.012
3.85
0.000
Pickup indicator
�164
0.011
�.90
0.000
Minivan indicator
0.302
0.011
28.50
0.000
Over-dispersion parameter
0.351
0.004
97.10
0.000
Log-likelihood Function for number of person-trips for all purposes
Constant only
�1,113.3
�0,951.3
Convergence
�7,983.1
�8,642.7
Pseudo-R2
0.026
0.076
Dependent variable: number of person-trips for recreational purposes
Constant
0.485
0.021
22.80
0.000
HH members per vehicle owned
0.204
0.008
25.90
0.000
Income per household member
�14E-06
4.73E-07
�60
0.000
Population density
�12E-06
1.09E-06
�60
0.000
Vehicle age
�018
0.001
�.60
0.000
Weekend day indicator
0.590
0.014
41.40
0.000
SUV indicator
0.005
0.021
0.20
0.814
Pickup indicator
�292
0.019
�.20
0.000
Minivan indicator
0.308
0.020
15.70
0.000
Over-dispersion parameter
1.009
0.011
93.40
0.000
Log-likelihood function for number of person-trips for recreational purposes
Constant only
�,460.37
�,6536.4
Convergence
�,612.10
�,753.51
Pseudo-R2
0.022
0.064
Number of observations: 41,538 vehicles
Model form: Number of person-trips ~ negative binomial with expected value
exp(β′X) and non-negative over-dispersion parameter
