there are a couple things that can be done to give warning that a machine is fixing to come through the doors

- install some type of motion alarm on the unit
- require the operator to blow horn when going through doors
- have floors marked as a caution area on both sides of the door labeled for operator to sound horn when going through that area
- in addition to the motion alarm a flashing beacon could be installed on the machine for visual warning

if you wanted to get really hi tech you could put sensors on the doors with flashing lights and a signal generator on the machine so when it got near the doors it would turn the flashing lights on and even sound an alarm.

there are all sorts of ways to give warning when a lift is coming through opening doors, it just depends on how much money you want to spend doing it :o)