(a) No person shall leave a vehicle unattended upon the vehicular crossing except in case of an emergency or in the conduct of business with the authority, customs officials, immigration officials, carriers or customs brokers, and then only in areas designated for such purposes.

(b) When it is permitted for a motor vehicle to stand unattended, no person driving or in charge of such vehicle shall leave it so unattended without first stopping the engine and effectively setting the brake thereon, and when standing upon any grade, turning the front wheels to the curb or blocking the wheels.