I. Introduction
Recently, autonomous driving has attracted significant academic, commercial, and popular interest. Since the 2005 DARPA Grand Challenge and the 2007 Urban Challenge, researchers have developed several successful technologies for autonomous driving applications. An often stated goal for developing the autonomous driving technologies is to improve driving safety, both to people inside and outside of the autonomous vehicle. To achieve this goal, the perception subsystem becomes a critical, first-line module in any highly autonomous driving architecture.