(4) Without prejudice to any other powers exercisable in that behalf, the Commissioner may give such directions with respect to the movement or, the route to be followed, by vehicular traffic, during such period, as may be necessary or expedient to prevent or mitigate congestion or obstruction of traffic or danger to or from traffic, in consequence of the holding of a race or trial of speed authorized under this section, including a direction that any road or part of a road specific in the direction shall be closed during any such period to vehicles or to vehicles of a class so specified.