(a) Except as herein otherwise expressly provided, no owner or operator of a heavy truck shall park or permit the same to be parked on any street or alley, at any time on Sundays and holidays, and between the hours of 9:00 p.m. and 7:00 a.m. on any other day.
(b) The provisions of this section shall not apply to a truck, tractor, or trailer which is parked while loading or unloading merchandise; nor to the parking of such truck, tractor or trailer during the time when the driver or operator thereof has stopped to eat, or for other necessary purposes, provided, however, that such parking shall in no event be for a longer period than thirty minutes; nor when such parking is necessitated by an emergency or by an act of God.