The Director is hereby authorized to regulate through truck traffic, and in the regulations prohibit trucks from using any highway or alley which has not been designated or maintained as part of the State or Federal highway system or an extension thereof, provided the Director has designated an adequate alternate route for truck traffic diverted or prohibited from using any highway or alley. When signs are erected giving notice thereof, no person shall operate any truck upon any such street, or portion of street, except that such vehicles may be operated thereon for the purpose of delivering or picking up materials or merchandise and then only by entering such street at the intersection nearest the destination of the vehicle and proceeding thereon no farther than the nearest intersection thereafter. This Section does not apply to light commercial vehicles.