BackgroundLow-calorie menu items as optimal defaults may encourage healthier choices when people eat out. Limited research has studied default effects from the restauranteurs' perspective, as well as the public health perspective.ObjectiveTo examine the effects of optimal defaults on calories ordered, dietary autonomy, and visit intention in the context of a fast-food drive-through.DesignBetween-subjects randomized scenario-based experiment.Participants/settingIn all, 377 adults who lived in the United States were recruited through a crowdsourcing platform in July 2020.InterventionParticipants were asked to visualize ordering a combo meal in a fast-food drive-through. They were randomly assigned to order from 1 of 3 menu boards: (1) menu items available for combos by customer choice, (2) combos that included traditional high-calorie default items, or (3) combos that included low-calorie optimal defaults.Main outcome measuresDifferences in calories ordered among groups, dietary autonomy, and restaurant visit intention.AnalysisStatistical tests included multiple regression, Kruskal-Wallis, χ2, and 1-way analysis of variance. Covariates such as education and sex were tested in regression models as potential confounders.ResultsCompared with the choice combo meals, optimal combo meals reduced calories ordered by consumers (−337 kcal, standard error = 19, P < .001), while traditional combos increased them (+132 kcal, standard error = 20, P < .001). No significant difference was found in visit intention. Dietary autonomy was affected by the optimal defaults (P = .025), even in participants with high health concern. Conversely, the traditional combo's effect on dietary autonomy was moderated by health concern (B = −0.26, P = .023), with only individuals with very high levels of health concern perceiving less autonomy.ConclusionsOptimal defaults provided a robust reduction in calories ordered but had implications for dietary autonomy.