University of California Transportation Center

The fundamental diagram is an important element in a variety of transportation studies. While various shapes of the fundamental diagram have been proposed and numerous debates on the best-fit fundamental diagram have been made, why the fundamental diagram has many different shapes has not been well explained. This study introduces time gap as a key parameter to understand drivers' behavioral differences at different locations and traffic conditions, then relate it to the shape of the fundamental diagram. From I-80 freeway event detector data, it is shown that time gap follows a certain probabilistic distribution and its mean value varies along locations. When downstream congestion is expected, drivers tend to take larger time gap than otherwise. It also turns out that drivers take different time gaps for different travel speeds. Three different types of time gap-speed diagrams are identified and matched to Greenberg, reversed-lambda, and inverted-V types of fundamental diagrams, respectively.