certain air-mile radius of their reporting location. State exemptions, however, are only applicable in intrastate commerce; stipulations of the Code of Federal Regulations (CFR) may be applied in interstate commerce. Also a person under the age of 21 cannot operate a commercial vehicle outside the state where the commercial license was issued. This restriction may also be mirrored by certain states in their intrastate regulations. A person must be at least 18 in order to be issued a commercial license. In addition, endorsements are necessary for certain cargo and vehicle arrangements and types; H – Hazardous Materials (HazMat or HM) – necessary if materials require HM placards. N – Tankers – the driver is acquainted with the unique handling characteristics of liquids tankers. X – Signifies Hazardous Materials and Tanker endorsements, combined. T – Doubles & Triples – the licensee may pull more than one trailer. P – Buses – Any Vehicle designed to transport 16 or more passengers (including the driver). S – School Buses – Any school bus designed to transport 11 or more passengers (including the driver). W – Tow Truck Role in trade Modern day semi-trailer trucks often operate as a part of a domestic or international transport infrastructure to support containerized cargo shipment. Various types of rail flat bed train cars are modified to hold the cargo trailer or container with wheels or without. This is called Intermodal or piggyback. The system allows the cargo to switch from highway to railway or vice versa with relative ease by using gantry cranes. The large trailers pulled by a tractor unit come in many styles, lengths, and shapes. Some common types are: vans, reefers, flatbeds, sidelifts and tankers. These trailers may be refrigerated, heated, ventilated, or pressurized, depending on climate and cargo. Some trailers have movable wheel axles that can be adjusted by moving them on a track underneath the trailer body and securing them in place with large pins. The purpose of this is to help adjust weight distribution over the various axles, to comply with local laws. Media Television 1960s TV series Cannonball NBC ran two popular TV series about truck drivers in the 1970s featuring actor Claude Akins in major roles: Movin' On (1974–1976) B. J. and the Bear (1978–1981) The Highwayman (1987-1988), a semi-futuristic action-adventure series starring Sam Jones, featuring hi-tech, multi-function trucks. Knight Rider, an American television show featured a semi-trailer truck