The Urban Mapping Mass Transit Proximity API enables users to enhance existing spatial databases, enrich location-based applications and promote alternative transportation options. This API exposes a subset of the extensive normalized data set maintained by Urban Mapping. Why? In many urban areas, critical decisions are often based on proximity to transit nodes such as subway stations, bus stops, commuter rail lines and airports. Similarly, access to public transportation can influence habits, routines, and tasks. However, data sources for mass transit system are highly idiosyncratic, difficult to extract, or unpublished. Urban Mapping solves these problems and offers normalized transit data that developers can use to create innovative applications.