Proposition 21 on California’s 2010 ballot concerned an annual surcharge on vehicles to support state parks. Proposition 21 failed, leaving 25% of California state parks at risk of closure. We analyze voting patterns, which we show depend on the average gross price of the proposition, political ideology, environmental preferences, the availability of substitutes, and park salience. We simulate counterfactual scenarios under which Proposition 21 might have passed and use holdout samples to illustrate the predictive ability of our model. Heterogeneity across California makes our model potentially useful for predicting public sentiment for similar propositions, even for jurisdictions without direct democracy.