It's not the 48kph that's the problem, it's the corner/ rock/ geriatric/ car that gets in the way that is the problem.
I think my fastest downhill speed was about 44mph (yes miles, I was in the UK), when I did the London to Brighton, I was on a touring treadly, I really like my road tourer, it's a dawes galaxy tour that I bought for £10 from a work coleague who pulled it out of a skip.