design idea - safety syringe

I've been playing around with ways to make simple everyday things safer.  Whilst looking at how needles are disposed I wondered why the lid couldn't be made into a disposing feature. 

When the lid is placed back over the needle and firmly pushed on it locks, making the needle safe and the two parts very difficult to separate.  The assembly can then be safely removed from the syringe and disposed and a new needle and lid fitted by pushing the syringe into the needle whilst holding onto its collar.