Adding timer to motor start push button.

[STEVEBOT]

While I agree with all the answers that have been given, my preference is that the horn is sounded, be it at intervals or continuous, while the operator is still pushing the start button, and at the end of that time period the circuit to run the motor is then latched. That way if the shout of someone in danger is given then removal of finger from the start button is all that is needed rather than having to hit the stop button.
Also if there is such a possibility occurring then consider using a castel coded key system where each person on the machine has to have a key and all the keys have to be returned before the start circuit can be energized.