Driving too slowly, especially on motorways, is dangerous. You're causing hundreds of people to change lanes. And lane changes are dangerous, for a variety of reasons, many of which BarracksSi detailed earlier. Not to mention that one car going slowly can tie up traffic for incredible distances. I've seen some very long queues due to slow drivers. Safe? I don't think so. Self-important? Depending on what, exactly it is you're doing, possibly.

Yes, driving is dangerous. But it's not just "OH MY GOD FAST!!!!" that is fatal. If you're doing something that is impeding the flow of traffic, like driving significantly slower than the flow, that's far from safe.