When the density of vehicular transit is high enough, vehicles cannot be regarded as point-like objects, in fact they require a finite space for vehicular extension and drivers reaction, such that vehicular length play a role. In the present work, the theoretical basis to construct the fundamental diagram is established from a kinetic equation in which those aspect are considered. We have obtained analytically the threshold density for which the fundamental diagram separates in two different regions: the individual and collective regimes. Since our model should be validated, we compared the results with Coifman's empirical data [1]. We found that both, the threshold density and the behavior of the fundamental diagram in the two regions show a successful agreement.

Acknowledgements. The authors acknowledge support from CONACyT through grant number CB2015/251273.

[1] B. Coifman. Empirical flow-density and speed-spacing relationships: Evidence of vehicle length dependence. Transp.Res. Part B, 78:5465, (2015).