(a) No person or persons shall leave a vehicle unattended on the vehicular crossing except in case of 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 those 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, effectively setting the brake thereon, and when standing upon any grade, turning the front wheels to the curb.