This indicates the third minor update or iteration released in 2021.

The core research paper associated with this specific version and dataset is:

The file is the primary download package for the Urban Traffic Dataset (UTD) , version 21.3. This dataset is widely used in academic research related to urban mobility, traffic flow prediction, and smart city infrastructure.

Could you clarify if you are having the zip file or if you need a specific citation for a research project?

Researchers use this data to train machine learning models for Traffic Prediction and Intelligent Transportation Systems. Where to Find Documentation