A recent systematic literature review found that primary enforcement laws are more effective at increasing seat belt use than secondary laws in the United States. This report re-examines the studies included in the systematic review to explore whether the benefits of a primary law differ based on: (1) the baseline seat belt use rate; or (2) whether or not the primary law replaces a secondary law. States that directly enacted primary laws showed larger increases in observed seat belt use (median increase of 33 percentage points). These laws were enacted in the mid-1980s, when baseline belt use rates were below 35%. Smaller, but substantial increases in belt use were observed in states that replaced secondary with primary laws (median increase of 14 percentage points). Baseline belt use rates in these states ranged from 47 to 73%. Primary safety belt laws can further increase seat belt use even in states with relatively high baseline levels of belt use.