Safe distances are determined by the distance it takes a train to come to a complete stop and the distance from a train to the one in front of it. The signal system does not enter into the equation.
But it does. The distance the following train needs to keep depends on its speed and its distance from the lead train. A block signal system uses only some of the data on the position of the two trains in question, and no data on speed.
- In a block system, the position of the lead train is only approximated. There is no difference whether the last few feet of the train are in a particular block, or the whole train. This difference can be reduced with smaller blocks, but smaller blocks cost more and aren't usually used.
- In a block system, the speed of the following train is irrelevant to the amount of space it must leave. Yes, block signals can demand that a train slow down. But they don't say: it's OK for you to go 45 mph here, but if you do you need to leave more space than if you go 20 mph.
So in theory, block systems could allow the same spacing as CBTC. In practice, a current NYCT block system will not allow as close spacing at full speed as a PROPERLY DESIGNED AND TESTED CBTC system.
This is not to say that NYCT's CBTC will be properly designed and tested. They have the potential to screw it up.