Transit passenger origin–destination flow estimation: Efficiently combining onboard survey and large automatic passenger count datasets

•Efficient method is proposed to estimate bus transit passenger route OD flows.•Combines large APC datasets and onboard survey OD passenger movement observations.•Extensive empirical evaluation is conducted against ground-truth OD flow data.•Computational feasibility and estimation accuracy are established.•Effects of APC and onboard survey sample sizes on accuracy are quantified.