Assume I could engineer and build a safety device that would create a safety zone around the danger, ie saw blade or other cutter. If ones body parts entered into the safety zone an alarm would sound and the device would shut down. No braking would be provided so danger still is present from the spinning blade/cutter. The safety zone could be set to the 3" rule or other specified distance. The alarm would possibly give the operator time to stop/pull back and in a commercial shop it would alert management to safety zone intrusions by employees.
This device could conceivably be added to most any stationary power cutting tool.
What if anything would such a device be worth?