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###[Appendix A. Derivation of a PDE describing traffic flow](id:sect-2.1.A)
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The purpose of this discussion is to derive a toy-model PDE that describes a congested one-dimensional highway (in one direction). Let
* $\rho(x,t)$ denote the traffic density: the number of cars per kilometer at time $t$ located at position $x$;
* $q(x,t)$ denote the traffic flow: the number of cars per hour passing a fixed place $x$ at time $t$;
* $N(t,a,b)$ denote the number of cars between position $x=a $ and $ x=b$ at time $t$.
It is directly implied by definition of $\rho(x,t)$ is
\begin{equation}
N(t,a,b)=\int_{a}^{b}\rho(t,x) dx.
\label{eq-2.1A.1}
\end{equation}
By definition of $q$ and conservation of cars we have:
\begin{multline}
\frac{\partial N}{\partial t}(t,a,b)=\lim\_{h \rightarrow 0} \frac{N(t+h,a,b)-N(t,a,b)}{h} \\\\
=\lim_{h \rightarrow 0} \frac{h(q(t,a)-q(t,b))}{h}
=q(t,a)-q(t,b)
\label{eq-2.1A.2}
\end{multline}
Differentiating (\ref{eq-2.1A.1}) with respect to $t$
\begin{equation\*}
\frac{\partial N}{\partial t}=\int\_{a}^{b}\rho_t(t,x) dx
\end{equation\*}
making it equal to (\ref{eq-2.1A.2}) we get the integral form of "conservation of cars":
\begin{equation\*}
\int_{a}^{b}\rho_t(t,x) dx=q(t,a)-q(t,b).
\end{equation\*}
Since $a$ and $b$ are arbitrary, it implies that $\rho_t=-q_x$. The PDE
\begin{equation}
\rho_t+q_x=0
\label{eq-2.1A.3}
\end{equation}
is conservation of cars equation.
After equation (\ref{eq-2.1A.3}) or more general equation
\begin{equation}
\rho_t+ \rho_x=f(x,t)
\label{eq-2.1A.4}
\end{equation}
(where $f=f\_{in}-f\_{out}$, $f\_{in}dx dt$ and $f\_{out}dx dt$ are numbers of cars entering/exiting highway for time $dt$ at the segment of the length $dx$) has been derived we need to connect $\rho$ and $q$.
The simplest is $q=c \rho$ with a constant $c$: all cars are moving with the same speed $c$. Then (\ref{eq-2.1A.3}) becomes
\begin{equation}
\rho_t+ c \rho_x=0.
\label{eq-2.1A.5}
\end{equation}
However more realistic would be $c=c(\rho)$ being monotone decreasing function of $\rho$ with $c(0)=c_0$ (speed on empty highway) and $c(\bar{\rho})=0$ where $\bar{\rho}$ is a density where movement is impossible. Assume that $q(\rho) = c(\rho)\rho$ has a single maximum at $\rho^\*$. Then
\begin{equation}
\rho_t+ v \rho_x=0.
\label{eq-2.1A.6}
\end{equation}
with
\begin{equation}
v=v(\rho)= q'(\rho) = c(\rho)+ c' (\rho)\rho
\label{eq-2.1A.7}
\end{equation}
where $'$ is a derivative with respect to $\rho$. Therefore $\rho$ remains constant along *integral line* $x - v(\rho) t=\const$.
$v=v(\rho)$ is the *group speed* namely the speed with which point where density equals given density $\rho$ is moving. Here $v(\rho)< c(\rho)$ (because $c'<0$), so group speed is less than the speed of the cars (simply cars may join the group from behind and leave it from its front). Further $v>0$ as $\rho<\rho^\*$ and $v<0$ as $\rho>\rho^\*$; in the latter case the group moves backward: the jam grows faster than it moves.
Also the integral lines may intersect (loose and faster moving group catches up with dense and slower group). When it happens $\rho$ becomes discontinuous, (\ref{eq-2.1A.3}) still holds but (\ref{eq-2.1A.6}) fails (it is no more equivalent to (\ref{eq-2.1A.3}) ) and the theory becomes really complicated.
->![traffic](./F2.1.A-1.svg)<-
**[Remark 1.](id:remark-2.1.A.1)**
In the toy-model of gas dynamics $c(\rho)=\rho$, or more general $c'(\rho)>0$ and $v(\rho)>c(\rho)$.
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