I started commuting by bike in London in June 2008, and it was still so light that I didn't need a rear light or a high visibility jacket after work to cycle back home. But now it's suddenly much darker, especially when it's cloudy or raining, so I started wearing a yellow high-viz jacket with a lot of reflective tapes around my back and arms - just like the one that the police officers are wearing on their motorcycle! I thought it'd improve my visibility a lot.
However, when I cycled I was carrying a dark coloured rucksack, and I realised about 70% of my yellow high-viz colour was covered by my rucksack. I've done some research when I was cycling behind other cyclists who were in the same situation, and the result was the high-viz yellow/orange colour + reflective tapes were hardly noticeable from behind in a distance if the cyclists were carrying a dark-coloured rucksack!
So I looked for a high visibility rucksack cover, and I chose this one. This particular item has two reflective tapes as well as a net pocket, so I put one of my rear flashing lights in it. Now I know the visibility has improved significantly, so I feel much securer behind my back. It has two rubber strings on the other side too, so it would stick to most rucksacks, I imagine. Only a small fault I can think of is that this cover is not waterproof, so your rucksack will probably get wet in the rain.
If you need to ride a bike after dark and carry a dark-coloured rucksack, I will definitely recommend this cover for you, especially if you cycle in heavy traffic like in London, and you can let others see you much sooner. Let's improve traffic safety and protect yourself. Good luck!
FYI: I can also recommend a waterproof
Hi-Viz Bomber Jacket to improve your visibility!