This paper contributes to the literature in two ways. First, the paper is the first to develop damage/casualty and frequency estimation model of vessel accidents incorporating ship speed. Second, the paper estimated accident frequency at a port level, which previous studies have not explored yet. Both models were complementarily used to explore the safety issue regarding vessel speed since the damage/casualty model alone could not explain the accident deterrent effect of speed reduction. The effects of speed reduction of vessels were examined by comparing accidents at RSZ ports with those at non-RSZ ports. The challenging task of separating accidents in RSZs was made possible using ArcGIS. The analysis was limited to a 7–20 nm range of ports because this range was effective to reveal the speed reduction policy.
At first, the results of the damage/casualty model showed that speed reduction intensified vessel damages and incurred more casualties per accident. To explain this unconvincing phenomena, accident frequency estimation was further conducted. In contrast to what damage/casualty model suggested, frequency estimation reported that speed reduction deterred vessel accident through two channels: the fuel price effect and the RSZ effect. Specifically, every $10 increase in the fuel price led to a 10.3% decrease in accident frequency. The effects of active RSZs on accident frequency was not apparent, which may be due to the small sample size and low compliance rates at the initial stage of implementing the policy. In inactive-RSZ ports, however, speed reduction effectively prevented accidents. Overall, RSZ programs reduced accidents by 48%. Combining two conflicting results, we concluded that potential accident with relatively small damages and casualty was avoided through speed reduction, leaving only impactful accidents in RSZ area. Nevertheless, whether speed reduction actually decreased damages or softened casualties was not clear.