Ladder collapse protector

  Having read about deaths occurring due to people extending ladders past the safe extension point I felt there had to be an answer.
  My idea is that the last rungs of the ladder have an extra extrusion on the back of the rung thus stopping the extended front part of the ladder from being located on the last rungs. This would stop people from overstretching the ladder thus avoiding a collapse due to undue stresses.
  An after market kit could also be sold as it would be quite a simple job to adapt old ladders.