A bottle on the parcel shelf, a laptop on the passenger seat, a toolbox in the boot: as long as the car is rolling, none of it is a problem. The problem appears at the exact moment it stops without warning.
What actually happens in a crash
When a car hits an obstacle, it stops over the length it is able to deform: about half a metre for a modern car at 50 km/h. The occupants, held by belts and airbags, stop with it.
Whatever is not attached carries on at 50 km/h. It will only stop when it meets something: a seat back, a headrest, someone's neck.
The arithmetic is not mysterious. Going from 13.9 metres per second to zero over half a metre is a deceleration of roughly twenty times gravity. An object therefore takes a force equal to twenty times its weight.
- A 1.5-litre bottle of water hits like 30 kg.
- A 2 kg laptop hits like 40 kg.
- A 5 kg bag of shopping hits like 100 kg.
- A 10 kg toolbox hits like 200 kg.
These are not brochure figures: it is elementary arithmetic, and it is why safety testing has long called for restraint nets behind the seats of estate cars.
The three places that cause trouble
The parcel shelf. The worst place in the cabin: nothing holds the object, it sits at head height, and it crosses the whole interior in a straight line. People still put tissue boxes, umbrellas and speakers there.
The passenger seat. An unused belt holds nothing at all, and a bag put there heads for the windscreen or the footwell — where your pedals are, including during a merely firm braking.
The boot of an estate or an SUV with no cover. With no net and no bulkhead, the boot is open to the cabin. A suitcase there has the whole length of the car to accelerate before meeting a seat back.
What works, in order
The general rule fits in one sentence: the heavier it is, the lower, further back and more wedged in it must be.
- The boot floor, against the rear seat backs. The safest position for anything heavy. An object pressed against the seat back cannot build up speed.
- A net or a bulkhead if you regularly carry volume, or if you drive with a dog.
- Hanging vertically for the soft bags of daily life. A headrest hook holds the bag against the seat back instead of leaving it loose on a seat. The load stays low, pressed flat, and out of the footwell.
- Nothing on the parcel shelf, ever. It is the cheapest habit to adopt and the one that changes the most.
A hook is obviously not an approved safety device, and we do not sell it as one: it keeps a bag from lying around and sliding under the pedals. To carry something heavy and hard, the only right answer is still the boot floor.
Children and animals
In the back, the danger rarely comes from a forgotten object: it comes from what we hand children during the trip. A tablet held at arm's length, a metal flask, a rigid toy. Prefer soft and light, and keep the rest in a seat-back pocket rather than on the bench.
For a dog, no amount of storage replaces a safety harness or a secured crate. An unrestrained twenty-kilo animal represents, in a 50 km/h crash, the equivalent of four hundred kilos thrown forward.
What to take away
There is no harmless object in a car, only objects that are well or badly placed. Walking round your cabin once, deciding on a place for each thing, and sticking to it: that is five minutes, once.