Research Article
Analysis of Severe Injury Accident Rates on Interstate Highways Using a Random Parameter Tobit Model
Table 2
Model estimation results.
| | | Fixed parameter model | Random parameter model | | | Parameter estimate | -ratio | Parameter estimate | -ratio |
| | Constant | | | | | | Logarithm of segment length | 1.015 | 20.651 | 1.033 | 17.528 | | Standard deviation of parameter distribution | NA | NA | 0.989 | 28.438 | | Logarithm of annual average daily traffic volume | 0.446 | 6.847 | 0.353 | 6.130 | | Standard deviation of parameter distribution | NA | NA | 0.303 | 14.646 | | Number of lanes per direction | 0.248 | 4.176 | 0.358 | 6.222 | | Standard deviation of parameter distribution | NA | NA | 0.288 | 7.727 | | Left shoulder width | | | | | | Standard deviation of parameter distribution | NA | NA | 0.024 | 3.308 | | Right shoulder width | | | | | | Standard deviation of parameter distribution | NA | NA | 0.016 | 3.000 | | Number of horizontal curves | 0.014 | 0.649 | | | | Standard deviation of parameter distribution | NA | NA | 0.032 | 2.955 | | Number of vertical curves | | | | | | Standard deviation of parameter distribution | NA | NA | 0.019 | 2.962 | | Number of observations | 589 | | Log-likelihood function at zero | −3,485.29 | | Log-likelihood function at convergence | | |
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NA: not applicable.
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