
Traffic flow is not that different from the flow of anything else. Imagine heat flow along an iron rod or water flowing down a pipe. If you observe the traffic (on a highway) from a far enough distance, the mass of cars would appear analogous to a (slowly) flowing fluid.
Let
be the density (substance per unit length) and let
be its rate of flow past x at time t.
The mass present in a segment of length
is:
![Rendered by QuickLaTeX.com \begin{equation} \[ \int_x^(x+\Delta x) \rho(x,t) \,dx \] \end{equation}](http://www.anujvarma.com/wp-content/ql-cache/quicklatex.com-c30a0c9277ebeb85899222ace7d17dc5_l3.png)
The rate of flow of this mass
![Rendered by QuickLaTeX.com \begin{equation} \frac{d}{dt} \[ \int_x^(x+\Delta x) \rho(x,t) \,dx \] \end{equation}](http://www.anujvarma.com/wp-content/ql-cache/quicklatex.com-33a94be016bda63c8572c8c6a17f332b_l3.png)