Vessel traffic places the sensitive environmental, natural, and cultural resources of Washington’s outer coast at risk of oil or hazardous materials spills. In 1995, International Maritime Organization established the Area to be Avoided off the Washington Coast (ATBA) to reduce this risk. Annual estimates of vessel compliance with the ATBA suggest that tugs towing oil and chemical barges have lower compliance rates than other types of vessel, but these compliance estimates have been contested. This project investigated some of the issues that have led to criticism of existing tug compliance estimates and examined several factors that could be related to observed cases of noncompliance. Analysis of vessel traffic and weather data from 2005 showed that: 1) the compliance rate for tugs towing oil barges increases by 8% to 91% and the compliance rate for tugs towing chemical barges increases by 21% to 57% when empty barge transits through the ATBA are treated as compliant instead of noncompliant; 2) most tugs towing laden oil barges through the ATBA did not pass deeper than one nautical mile into the Area while tugs towing laden chemical barges through the ATBA all passed deeper than one and a quarter nautical miles into the Area; 3) a large portion of transits involved tugs calling at ports on the Columbia River; 4) there is inconclusive evidence of an association between tug passage through the ATBA and use of the Two-Way Route in the Strait of Juan de Fuca; 5) there is virtually no evidence of an association between tug compliance and weather; and 6) most tugs inbound for the Strait of Juan de Fuca were towing empty barges. A review of incident and accident data showed that, between 1994 and 2006, an average of one incident or accident involving a tug towing an oil or chemical barge occurred off the outer coast each year. These findings should help guide decisions about prioritizing spill prevention efforts along Washington’s northern outer coast.