Landing Gear Retracting legs which support the front of a semitrailer when it is not coupled to a tractor.

Layover Any off-duty period away from home.

LCV (Long Combination Vehicle) In general, vehicles longer than a standard doubles rig (tractor and two 28ft semi trailers).
Examples of LCVs which are permitted in some U.S. western states and eastern toll roads: Twin 48ft trailers; triple 28ft trailers.
Lessee Company or individual which leases vehicles.
Lessor Company which leases vehicles.

Lift Axle Extra, unpowered axle needed only when the vehicle is loaded, allowing it to meet federal and state vehicle weight standards. The lift axle is mounted to an air spring suspension that raises the axle when it is not required.

Line haul The movement of freight from one point to another. It does not include pickup and delivery services

LPG Liquefied Petroleum Gas, e.g., Propane Butane.
Logbook Book carried by truck drivers in which they record their hours of service and duty status for each 24-hour period. Required in interstate commercial trucking by the U.S. Department of Transportation.  
Lowboy Open flat-bed trailer with a deck height very low to the ground, used to haul construction equipment or bulky or heavy loads.

LTL (Less-Than-Truckload) A quantity of freight less than that required for the application of a truckload (TL) rate; usually less than 10,000 pounds.

LTL Carrier Trucking company which consolidates less-than-truckload cargo for multiple destinations on one vehicle.