Time-varying and non-linear associations between metro ridership and the built environment

•Temporal heterogeneity exists in the associations between metro ridership and the built environment.•Built environment variables have non-linear and threshold effects on metro ridership.•Distance to the city center and population density are strong predictors of metro ridership in the morning peak hour.•Employment density, enterprise density, and road density are strong predictors in the evening peak hour.•Critical TOD planning parameters are identified.