Enough braking distance is provided to allow a train to comfortably stop at the stop signal, but should it fail to do so for any reason (fog, smoke, slippery rails, brake failure, inattention by driver, etc.)
The length of an overlap may be a nominal length, or else calculated on the full emergency braking distance for the gradients and speeds for that particular section of line.
However, overlaps may then be longer than is necessary with an adverse effect on headway.
Overlap design vary widely with signalling practice in different countries and types of train protection systems.
In Germany the following distances before entry to a station or another critical point are used:[1] (Typical values)