The causal effect of monthly pass programs on public transportation and carbon emissions

While existing research has examined the relationship between monthly pass and public transportation usage, limited attention has been given to their effects on carbon emissions. This paper evaluates the impact of a monthly pass program in the Taipei Metro system on travel behavior. Using administrative origin–destination records and a difference-in-differences method, we find that the program increased metro ridership, passenger kilometers, and trip value by 4.3%, 4.6%, and 4.6%, respectively. The effects are more pronounced for trips occurring on weekends, during peak hours, and at stations located near schools, residential areas, and business districts. Moreover, a larger effect of the program is found at higher ticket prices. Estimates of a back-of-the-envelope calculation suggest that the program delivers negative overall carbon emissions as long as at least 70–80% of the increased metro usage was shifted from trips previously made by private cars or motorcycles.