Movement of 50-70 per turn so maybe 0.6% chance of tripping per move point used? Only a couple AP to stand up but instantiates current move points. Hardly affects moving straight line but slows those moving far for stairs to come back above start position for no MP cost. That % is about 25% cumulative for moving 60 but 45% for equivalent of 120. 1 trip max per turn. Could happen early, middle, end but usually the middle or end would be the worst. Could bypass check for moves less than 50 to lessen effect on simple moves.
Also need cost for moving verticle on steps/ramps at least 5 MP per floor. Something...