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.