"Is dangerous cycling a problem" BBC

But then what is 'fast' on a bike? To some 15mph is considered fast but to me it's something I would think of as moderate to slow.

I don't think "fast" is the appropriate descriptor, here. "Unsafely" would be better (and correct use of an adverb), and will depend on skill. But basically, you should be able to slow considerably, or stop, under the power available to you at the time (legs and/or brakes), without causing significant damage to yourself, your vehicle, or anyone else, should something unexpected come up.

Then again, I don't ride like that. But I should.