Intelligent transportation systems (ITS) are deployed around the world to improve the safety and efficiency of surface transportation through the application of advanced information technology. However, ITS expose the transportation system to new vulnerabilities, making it imperative that those implementing such systems design them to be more survivable. The approaches most commonly used to increase the survivability of ITS are physical security, hardware redundancy, and software and data backups. An essential element of a survivable system that has not been adequately addressed is intrusion detection. Intrusion detection seeks to identify individuals who have gained unauthorized access to a system, as well as those with authorized access who are abusing the system. Described is the importance of robust intrusion detection for ITS (and other transportation information systems). The state of the practice in intrusion detection is summarized, and a case study is presented that investigates the development of an intrusion detection system (IDS) conceptual design for an essential element of ITS, electronic toll collection. Through the case study, the researchers found that intrusion detection at the application level could significantly improve the overall survivability of ITS. From this finding, it is clear that there is a need to commit resources to developing application level IDS for the wide range of ITS. Given the importance of surface transportation to a region's economy and the potential vulnerabilities introduced by ITS, it is imperative that further work be conducted to increase the survivability of ITS.