Outside stairs and safety

I'm not sure if this is really an appropriate question for here, but I'd be grateful of advice!
There's a flight of external stairs that people have tended to fall over on over the years. I've suggested better grip in the wet might be a good idea and, as a result, an exterior paint, mixed with sand, has been applied, giving a much better grip.
People still fall over - the stairs aren't even outside a pub, so that's not the explanation!
I had somebody criticise some other stairs, saying that the risers were too high and might be dangerous, so I wondered if that might be part of the problem.
These stairs have a railing, but the riser hight is 17cm and the tread width 28cm. I gather, from a brief look at things, that 15cm is a good size for a riser. Is 17 cm too high? Would it make sense to put an extra stair between each of the current ones giving 8.5 and 14 cm, or would that make it worse?
Not everybody using the stairs is in the first flush of youth, which is part of the problem, I know. They have a railing, on the right hand side going down, but they are over three metres wide. Should they have a railing down the middle? Is the height of the railing important?
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