Imagine an aeroplane cabin with no overhead storage. The aisle would be blocked, leg room cramped, and if the plane hit turbulence – well, things could get ugly. It may seem like a far-fetched comparison, but it's not unlike what many people do when it's time to pack the car for a holiday. If it can't be stuffed into the boot, chances are it will end up in a footwell, on a seat or in someone's lap. And the consequence can be a long, uncomfortable and potentially dangerous journey. However, there is a way to add some extra storage to your car.

Think inside the box

They don't come cheap but if you're a regular camper or like to head off on weekends away, a car roof box (also known as a luggage box or roof pod) can definitely be worth it.

A roof box fits onto the roof bars of your car and can add an extra 300 to 600 litres of storage space. But more importantly, they make a long drive safer. They improve your rear visibility (if you tend to pile things up in the boot or back seat) and reduce the risk of objects flying around the cabin in the event of an accident.

Getting attached

Roof boxes fix to your roof racks either with a basic U-bolt system or a "claw"-style attachment. The claw attachments make it easier and quicker to mount and remove the box.

Keep in mind that your car uses more fuel when it has a roof box attached – our test car used 12.1% more fuel with the roof box fitted than without. So you really don't want to leave the box on when it's not being used. If it's easy to remove, you'll be more inclined to take it off sooner rather than later. The top two roof boxes in our test were both very easy to fit.

What else do I need to look for?

Compatibility

The roof box must be compatible with your roof bars. Check the shape, width and distance between your bars.

Size and shape

A roof box that's too big or the wrong shape can interfere with rear hatch or wagon opening. Narrow boxes can be difficult to reach, particularly on wider or taller vehicles. There are a range of shapes and sizes on the market, some with more specialised designs if you need to carry skis or other sporting equipment.

Opening

Boxes that open from both sides offer the most flexibility and practicality. Boxes that open from the driver's side only present a hazard if you can't pull off the road.

Load rating

Roof boxes have load ratings. Make sure you choose one that will safely hold the weight of your luggage.

Loading

Put heavier items in the boot of your car and use the roof box for lighter items. This will have less impact on your car's centre of gravity and also make items easier to stow and retrieve.

Cost

The models we tested ranged in price from $449 to $829.