I jet wash mine in spring and autumn, just done the autumn one now and unless its wet (wet floor is always slippery right?) its not slippery. As to pressure washing damaging the decking, yes you can if you put the pressure washer on to high a setting and to close, but pressure washing is absolutly the recommended way to do it. less than 1300 PSI will not damage decking.
Other options, you can buy grip sticks, which you nail onto the slippery areas, these in my opinon dont look very good, but they server a purpose and may be a better option for a curved bridge (assume its higher in middle than side?)
Second option is gripdeck, these are small inserts that you buy and slot into some of the grooves in the decking boards. They are not cheap, but they look the part and work well.
Last opition, is to replace the deck boards with the anti slip boards, these are sold in b&q and im sure elsewhere for a more reasonable price, they are a standard decking board with anti grip stuff already attached.
Hope these help.