Sorry, I realize this isn't the advice you were asking for...however, this is a common problem when the wrong sort of pavers are used. My uncle is a mason who has done dozens of beautiful stone and brick sidewalks for people, but according to him you have to choose the right pavers because a lot of them may be rated for outside but are not designed to prevent slipping and falling. Apparently there is something he looks for when selecting the right pavers for the job. A lot of things like slate can be extremely slippery when wet and many natural stones will ice up in the winter. I'd have to ask him what that specific thing is that he looks for, but when someone hires him and ignores his advice to use a different paving stone he'll make them exactly what they asked for...and has frequently had them call him back to replace it with something safer.