No tank vehicle containing flammable, combustible,
corrosive, poisonous, toxic or hazardous liquids, gases or residues
of the same shall be left unattended on any street, highway, avenue
or alley, provided that this shall not prevent a driver from the necessary
absence from the truck in connection with the delivery of his load,
except that during actual discharge of the commodity, some responsible
person shall be present at the vehicle, nor shall it prevent stops
for meals during the day or night if the street is well lighted at
point of parking.