1. Introduction
Motion forecasting has received increasing attention as a critical requirement for planning in autonomous driving systems [8], [14], [40], [36], [28], [34]. Due to the complexity of scenes that autonomous systems need to safely handle, predicting object motion in the scene is a difficult task, suitable for machine learning models. Building effective motion forecasting models requires large amounts of high quality real world data. Creating a dataset for motion forecasting is complicated by the fact that the distribution of real world data is highly imbalanced [4], [18], [32], [38]; in the common case, vehicles drive straight at a constant velocity. In order to develop effective models, a dataset must contain and measure performance on a wide range of behaviors and trajectory shapes for different object types that an autonomous system will encounter in operation.