Is it dangerous to perform a forward roll when you have something on your back, assuming that one performs it correctly?
For example, if you have a backpack with various hard things, like books or boxes inside, and you trip or get pushed, is it better not to roll or does it even matter?
My advice is to just try it. You probably have a backpack. And you probably know how to do a front shoulder roll. Give it a try. See how it changes the way things work. Ask yourself whether or not you can modify it to roll better.
Safety is a concern always, no matter what type of breakfall you're performing. So approach things slowly, with little extra weight at first. Then increase the intensity when you can do so safely. If you ever feel like your back, shoulder, or neck are getting torqued or flexed in a direction they shouldn't be flexed, stop and don't try it again until you've worked out what went wrong. The backpack might change your center of gravity, which affects how you perform the breakfall. And the straps might end up riding up and strangling you or preventing your shoulder from flexing the way it needs to. You won't know until you try, which is why you need to go slow and with little weight at first.
"Dangerous" is relative. In most cases, you won't sustain much harm by doing so, although the items in your backpack might get crushed. I have had it explained to me before by my former teachers (jujitsu and Bujinkan) that this is one of the reasons some of the defenses involve turning a flip to escape from a hold rather than a roll, because it prevents you from rolling over your sword. At that, a breakfall may also be a better choice in some situations. Learning multiple options lets you react appropriately for the situation.
I suppose it might depend on the content of your backpack too. Breakables, sharps, and objects with lumpy corners could dig into your spine or organs close to the surface. It's probably about the same risk that you'd face if the contents of your backpack were on the ground and you rolled over them.
They make parkour backpacks that are designed to be able to be rolled over. Unfortunately the pockets are tiny.
If you've got things like say, clothes in your backpack, it would make your roll softer.
Edit: Check out this link: http://ultimateparkourgear.com/parkour-backpacks/ You can also try wearing your backpack backwards XD
My advice would be like Steve Weigand's: give it a try. But start by putting something soft into your backback, like a pillow or some folded towels, and also be conscious of the possibility that clips or zip-pulls on your pack could damage the surface of tatami.
Next, try and get a sense for the direction in which you are rolling. For instance, in practice, mae ukemi is normally from one shoulder to the opposite hip. In that case, all your weight would roll across the backpack. However, you may sometimes have done a rather more "sideways" ukemi where you roll "up one arm, across your shoulders, and down the other arm"; in that case, less of your weight would roll across the backpack. Also, Donovan Waite Shihan has a number of excellent Youtube videos demonstrating ukemi - though he sets a higher standard than I can aspire to.
I once got caught out by the sudden braking of a bus, and momentarily considered mae ukemi down the aisle of the vehicle... but since I was wearing a backback and it had my laptop in it, I just dropped to all fours. It wasn't elegant, but I'm sure ukemi practice meant that I had time to adjust my reaction appropriately.
Hope this helps.
