The flow-density relationship for traffic is $q(\\rho) = u_{max} \\rho (1 - \\frac{\\rho}{\\rho_{max}})$, where $u_{max} = 100$ km/h (free-flow speed) and $\\rho_{max} = 120$ vehicles/km (jam density). A traffic jam has density $\\rho_2 = 100$ vehicles/km, while upstream traffic has density $\\rho_1 = 40$ vehicles/km. What is the propagation speed of the shock wave at the boundary between these two states?