Since 1993, photoelectric sensors (aka photo-eyes) have been required on garage door openers. The CPSC recommend this change to “reduce the number of deaths to children who become entrapped under garage doors with automatic openers.” I’m not saying it’s a coincidence, but after watching Indiana Jones roll under that temple door in 1981, it pretty much became obligatory for my brother and I to start diving under closing garage doors. While I don’t think you can blame the Barraza boys for this change, it may well have been our generation that prompted it.
How they work
Photo-eyes work by sending an infra-red beam between the two sensors, and if something were to interrupt the beam while the garage door is closing – the opener will reverse and open again. The sensors need to be properly aligned for the system to work, and some crafty (aka devious) folks have figured out ways to defeat this safety feature. (See photo below)