This state-of-the-art report is the first of two reports. Both are the result of a Swedish project, the Swedish Energy Authority’s TENS project, which spanned from 2018-2021. This report lays out the background, related work, state of the art, related projects as well as mapping out the area for the newcomer to the area. Examples are from Sweden due to the available data, as well as the locally installed infrastructure. We dedicate some space to the data gathering process as well as basic traffic flow principles. It should be seen as introductory material to the topic, getting the laymen up to speed in data processing issues arising from using traffic sensors. A second (sister) report contains technical results from our work in the above named Swedish project.