Not that law enforcement really cares about these kinds of issues, usually claiming they are to busy, here is the real law. 42-4-1206.
 Unattended motor vehicle
No person driving or in charge of a motor vehicle shall permit it to stand unattended without first stopping 
the engine, locking the ignition, removing the key from the ignition, and effectively setting the brake 
thereon, and, when standing upon any grade, said person shall turn the front wheels to the curb or side of 
the highway in such a manner as to prevent the vehicle from rolling onto the traveled way. Any person who 
violates any provision of this section commits a class B traffic infraction.