In this paper, we discuss an application of spatial data mining to predict pedestrian flow in extensive road networks using a large biased sample. Existing out-of-the-box techniques are not able to appropriately deal with its challenges and constraints, in particular with sample selection bias. For this purpose, we introduce s-knnapriori, an efficient nearest neighbor based spatial mining algorithm that allows prior knowledge and deductive models to be included in a straightforward and easy way. Categories and Subject Descriptors I.2.6 [Learning]: Induction, Knowledge acquisition; I.5.1 [Models]; I.5.2 [Design Methodology] General Terms Algorithms, Economics, Reliability, Human Factors, Verification Keywords Spatial data mining, pedestrian flow prediction, sample selection bias, prior knowledge, extensive road networks, large scale data