What's half a car length?
Depends of course on the car.
Could be a metre or two.
But judging from the look on the face of the driver of a red Acura Integra I saw on the 401 the other day, he wishes he had left more than that between himself and the car in front of him.
Because from my vantage point (the adjacent lane) it appeared as if he had stopped very close to said car, and when the transport truck nailed him from behind, both ends of his little car got squashed.
A good rule of thumb is to always stop at least far enough behind the car ahead of you so you can see where his rear tires meet the pavement.
Not only might this prevent someone from shoving you into the back of the car in front, it might just give you enough room to get the heck out of there if you're paying enough attention to notice the transport truck bearing down on you, tires locked and smoking.