  The main problems in transportation are accidents, increasingly slow traffic
flow, and pollution. An intelligent transportation system (ITS) using external
infrastructure can overcome these problems. For this reason, the number of such
systems is increasing dramatically, and therefore requires an adequate
overview. To the best of our knowledge, no current systematic review of
existing ITS solutions exists. To fill this knowledge gap, our paper provides
an overview of existing ITS that use external infrastructure worldwide.
Accordingly, this paper addresses current questions and challenges. For this
purpose, we performed a literature review of documents that describe existing
ITS solutions from 2009 until today. We categorized the results according to
technology levels and analyzed its hardware system setup and value-added
contributions. In doing so, we made the ITS solutions comparable and
highlighted past development alongside current trends. We analyzed more than
357 papers, including 52 test bed projects. In summary, current ITSs can
deliver accurate information about individuals in traffic situations in
real-time. However, further research into ITS should focus on more reliable
perception of the traffic using modern sensors, plug-and-play mechanisms, and
secure real-time distribution of the digital twins in a decentralized manner.
By addressing these topics, the development of intelligent transportation
systems will be able to take a step towards its comprehensive roll-out.
