Final answer:
Compressed natural gas or propane vehicles need to have a decal on the rear portion, helping to identify them easily for safety and refueling purposes.
Step-by-step explanation:
Vehicles powered by compressed natural gas or propane are required to have a decal or sticker on the rear portion of the vehicle and adjacent to refilling locations. This requirement is in place to ensure that the vehicles are easily identifiable by emergency responders and service personnel. The decals also serve to indicate that the vehicle is fueled by an alternative fuel source, which often has different handling and storage requirements compared to gasoline or diesel.