This guide presents a primer on digital twins in the transportation sector, detailing their definitions and applications. It explains three forms of digital representations: digital models, digital shadows, and digital twins. A digital model serves as a static snapshot of a physical object or process, while a digital shadow maintains a live data link, allowing for real-time updates without feedback to the physical entity. The digital twin, however, features two-way data flow, enabling it to interact with the real world by both receiving and sending data. The document outlines the potential benefits of digital twins in optimizing transportation systems, such as variable messaging signs that can respond to real-time traffic conditions. It also discusses challenges in data integration and the importance of starting with a limited scope for effective implementation. The guide emphasizes the transformative potential of digital twins in enhancing transportation management and decision-making processes.