A reefer trailer holds a setpoint temperature, either running continuously or cycling on and off, and logs the temperature throughout the trip. The load is only as good as that temperature record — a break in the cold chain can condemn the whole shipment.
Reefer freight carries more operational risk than dry van: the unit can fail, produce generates its own heat and needs airflow, wash-outs are required between incompatible commodities, and detention at packing houses and grocery DCs runs long. Rates are correspondingly higher.
Reefer trailers also haul dry freight when refrigerated volume is thin, which is one of the ways capacity shifts between segments.
Example
A carrier loads fresh strawberries at a cooler in California at a setpoint of 34°F, with the reefer on continuous run and a temperature recorder in the load. At the grocery DC four days later, the receiver checks the download before signing — a clean record means the load is accepted; a excursion above spec can mean rejection.
Why it matters
On a reefer load the temperature record, not just the POD, decides whether the carrier gets paid. Dispatchers price reefer lanes with the extra detention, wash-out, and claim risk built in.