  Digital twin (DT) systems aim to create virtual replicas of physical objects
that are updated in real time with their physical counterparts and evolve
alongside the physical assets throughout its lifecycle. Transportation systems
are poised to significantly benefit from this new paradigm. In particular, DT
technology can augment the capabilities of intelligent transportation systems.
However, the development and deployment of networkwide transportation DT
systems need to take into consideration the scale and dynamic nature of future
connected and automated transportation systems. Motivated by the need of
understanding the requirements and challenges involved in developing and
implementing such systems, this paper proposes a hierarchical concept for a
Transportation DT (TDT) system starting from individual transportation assets
and building up to the entire networkwide TDT. A reference architecture is
proposed for TDT systems that could be used as a guide in developing TDT
systems at any scale within the presented hierarchical concept. In addition,
several use cases are presented based upon the reference architecture which
illustrate the utility of a TDT system from transportation safety, mobility and
environmental applications perspective. This is followed by a review of current
studies in the domain of TDT systems. Finally, the critical challenges and
promising future research directions in TDT are discussed to overcome existing
barriers to realize a safe and operationally efficient connected and automated
transportation systems.
