Once in effect, a prohibition under this chapter shall remain in effect for 48 hours unless sooner terminated by the Mayor or the President of Council, except that any street area which has become substantially clear of snow and ice from curb to curb for the length of the entire block shall be automatically excluded therefrom. While the prohibition is in effect, no person shall park or allow to remain parked any vehicle on any portion of the snow emergency route to which it applies. Owners of vehicles already parked at the commencement of a snow emergency shall have two hours in which to remove their vehicles. However, nothing in this section shall be construed to permit parking at any time or place where it is forbidden by any other provision of law.