A seedling loaded into a vehicle is alive for the whole journey; it respires and produces heat and moisture. According to the FAO manual on fresh produce, rapid respiration leads to rapid heat build-up. In a body where air does not move, the middle of the load is always warmer than its edges, so what has to be measured is not only the degree but the airflow.
With bare-root stock, the roots are kept moist, the load is shielded from wind and direct sun, and bundles are stacked loosely for ventilation (Penn State Extension). Write species, root type, loading time and temperature on the dispatch form, and match leaf, shoot and root condition on arrival to the same batch.
The body is a variable too. A volume that passes heat slowly instead of a steel body heated directly by the sun, loading in shade, and air intakes that let air move through the load buy hours on a hot day; FAO also lists refrigerated vehicles for long journeys, with a note on their cost.