Loading is the moment a shipment takes on the most heat: the dock is open, the goods are outside and the body has warmed in the sun. When all three meet at midday in summer, the first hour of the journey goes on recovering the loss. At night the solar load is gone and the air and tarmac cool down; the same goods enter the same body at a lower temperature.
For the coolness to be really free, the rest of the system has to fit. A night shift costs money and store receiving hours are limited. A passive container closes that gap: eutectic plates are frozen overnight, the container is prepared overnight and the shipment leaves in the morning. The cold is made at night and delivered by day; if your electricity tariff varies by time, the production also falls in the cheaper hours.
A small trial
Run one line on night loading for two weeks. Measure product temperature at loading and arrival with the same method, and record unit running hours, how long the door stayed open and the shift cost. Compare with day trips of the same product, load, vehicle and route, and take several matched trips, because a single day is affected by weather and traffic.
