A vast amount of literature exists on the fundamental speed-density_ flow models of traffic theory and various travel time (cost)-flow models used for transport planning and demand management purposes. There is often a confusion about how these two groups of models are related, especially in terms of congested traffic conditions. This paper offers an explanation of the relationships between speed, density, flow and travel time models for uninterrupted and interrupted traffic flows.
AKÇELIK, R. (1996). Relating Flow, Density, Speed and Travel Time Models for Uninterrupted and Interrupted Traffic. Traffic Engineering and Control 37(9), pp 511-516.