A car is the hardest vehicle for chilled transport and the most commonly used one: there is no air circulation in the boot, the rear window lets the sun in, and the temperature climbs within minutes once the engine stops. Keeping the cabin cool with air conditioning does not keep the boot cool, so what protects the product is the container around the load, not the vehicle.
Choosing the container. The loaded container must pass through the boot opening, must not block vents once secured, and must be light enough for one hand; a courier is usually juggling a phone, paperwork and keys at the same time. The lid should open and close in one movement, because a latch that needs two hands at every stop ends the day left open. Keep ice packs from touching the product directly, and choose a container that suits the product's target range.
On the round. Two drivers get different results from the same container. One who throws the lid fully open at every stop, leaves the container on the passenger seat in the sun and parks in the open instead of in shade at lunchtime gives back the hours the container earned. Putting the next order on top cuts lid-open time to seconds, and writing down departure, stop and final delivery times makes the round readable afterwards.
Where the limit is
On a trial round, place a data logger beside the product inside the pack and read the full record on return. One good result does not guarantee a different load, weather or route. If the record leaves the target range, shorten the round, cut stops or move to a temperature-controlled vehicle; there is no universal cutoff by container count or number of stops.
