Intelligent Transportation Systems (ITS) have transformed modern transportation networks by integrating advanced technologies, communication systems, and data analytics. However, this interconnectivity has introduced a new set of challenges related to cybersecurity. Intrusion Detection Systems (IDS) play a pivotal role in safeguarding ITS from cyber threats. This journal article explores the realm of ITS intrusion detection, covering challenges, trends, practical implementations, evaluation metrics, regulations, and collaborative efforts. The introduction establishes the significance of IDS within ITS, emphasizing the vulnerability of interconnected transportation networks. The subsequent sections delve into the intricacies of ITS and IDS, highlighting the role of IDS in detecting anomalies and potential security breaches. The challenges section addresses issues such as high data volumes, dynamic environments, and real-time processing requirements, while the future trends section envisions the impact of threat intelligence sharing, context-aware detection, IoT integration, and blockchain. Real-world case studies exemplify successful IDS implementations in ITS, offering lessons learned and insights into performance evaluation across different scenarios. Evaluation metrics, both traditional and specialized, guide the assessment of IDS effectiveness. The landscape of regulations and standards is explored, from government guidelines to industry norms, underscoring the importance of aligning ITS security with general cybersecurity frameworks. The conclusion underscores the collaborative efforts required to ensure the security of ITS. By recapitulating challenges, trends, and the road ahead, it reinforces the need for unity in the face of evolving threats. In a landscape where transportation systems are increasingly intelligent and connected, robust intrusion detection measures are crucial to maintaining the integrity, safety, and resilience of ITS networks.