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Homework Statement
This is exercise 2.4.2 from Pringle and King, Astrophysical Flows (CUP 2007):
Along an infinite, straight, one-track road the local density of cars is [itex]\rho(x,t)[/itex] and the local velocity of cars (all assumed to be traveling in the same direction) is [itex]v(x,t)[/itex]. Discuss why it might be reasonable to assume that [itex]v[/itex] is solely a function of [itex]\rho[/itex].
Making this assumption, show that [tex]
\frac{\partial \rho}{\partial t} + c(\rho) \frac{\partial \rho}{\partial x} = 0
[/tex] where the kinematic wave speed is defined by [itex]c(\rho) = Q'(\rho)[/itex] and [itex]Q = \rho v[/itex] is the local flux of cars.
Traffic flow along a particular highway can be fitted approximately for [itex]\rho < \rho_{\mathrm{max}}[/itex] by [tex]Q(\rho) = V_0\rho \log(\rho_{\mathrm{max}}/\rho),[/tex] where [itex]V_0 = 25\,\mathrm{kph}[/itex] and [itex]\rho_{\mathrm{max}} = 150\,\mathrm{vechicles}\,\mathrm{km}^{-1}[/itex].
Show that information propagates upstream at a speed [itex]V_0[/itex] relative to the local vehicle velocity.
Show that there is a maximum traffic flow which occurs at some density [itex]\rho_{\mathrm{crit}}[/itex] corresponding to a critical speed [itex]v_{\mathrm{crit}}[/itex] of around 75 kph.
[Remainder omitted]
Homework Equations
The Attempt at a Solution
All is straightforward until we consider the particular choice of [itex]Q[/itex]. Firstly we have [tex]
c = Q' = V_0\left(\log(\rho_{\mathrm{max}}/\rho) - 1\right) = \frac Q\rho - V_0 = v - V_0[/tex] so indeed [itex]c - v = -V_0[/itex].
Now we're asked to find the maximum traffic flow. This corresponds to the maximum of [itex]Q[/itex], and we've just shown that [itex]Q' = v - V_0[/itex]. Thus the maximum occurs at [itex]v_{\mathrm{crit}} = V_0[/itex], at which point [itex]\rho_{\mathrm{crit}} = e^{-1}\rho_{\mathrm{max}}[/itex].
According to the question, [itex]v_{\mathrm{crit}} \approx 3V_0[/itex]. Am I missing something obvious, or is the question wrong?