3. Saw Them
A rough square edge could cut through zip tie plastic, but it takes some time. Brick, concrete, stone, and cinderblock edges work well; however, these square edges might not be readily available in a captive situation. This is where you need to be prepared with a back-up plan. By swapping out your ordinary boot or shoe laces with 550 cord (or Kevlar cord) when you first start wearing them, you'll always have several feet of remarkably useful cordage material available. To use the 550 cord to cut through zip ties, start by removing the laces from both shoes. Tie them together and then tie a large loop at either end of your extended cord (a bowline is the perfect knot for the loops). Each loop should be large enough to slide over your foot to the arch. Thread your cordage under one of the zip ties, put a foot in each loop and start moving your feet to make the cord slide back and forth. A "bicycling" motion works best, and keep the cord in the same stop on the zip tie. The 550 cord is resistant enough to the friction heat you'll generating, but most zip tie material should melt and wear away under this friction. Bicycle as hard as you can, and the ties will soon be cut. Re-lace your footwear, and be ready for escape and evasion.