OPA 90 set out stringent requirements and liabilities for tankers operating in US national waters. OPA 90 was in response to the public concern caused by the grounding of the Exxon Valdez in 1989. It made ship owners responsible for the cost of pollution incidents and required all tank ships/barges operating in US waters have double hulls by 2015. We model factors influencing oil spills and test whether OPA 90 helped reduce the number of those spills. After accounting for causal factors, both increased liability and double hulls were statistically significant factors in reducing the number of spills.