I believe there is an AAR formula for determining signal spacing, but the bottom line is the block signal spacing has to be arranged to provide sufficient stopping distance.

The basic signal system uses three aspects - red, yellow and green. As trains got heavier or speeds increased, requiring a greater stopping distance, more aspects can be added.
For example:

Green - Clear, proceed at maximum authorized speed.
Flashing Green - Limited Clear, proceed not exceeding 50/60 mph
Flashing Yellow - Advance Approach, proceed prepared to stop at second signal, trains reduce to 30/40 mph before passing next signal.
Yellow - Approach, Immediately reduce speed to 30/40 mph prepared to stop at next signal.

Flashing Green is fairly rare, but Flashing Yellow or a similar aspect with the same definition are common. A train might receive one, two or three blocks of warning before a red signal depending on the system. An alternative method is to restrict certain trains by rule to insure they can stop in time, either by restricting their maximum speed over a territory or approaching certain signals.

There are instances where signals are close together, typically where crossovers, junctions, sidings, beginning or end of another main track, etc. are close to each other. In these cases, the system will be wired to provide two or more yellow signals in advance of the actual stop signal to provide sufficient advance warning.

I did work as a temp conductor on one railroad that had a section of CTC that must have been a Kmart clearance item. Because the original traffic density was fairly low, the intermediates were about a mile before an absolute at a siding, but the rest were miles apart. Their signal rules for a yellow/approach required prepared to stop at next signal not exceeding 40 mph, but the track speed was 40 mph. The intermediates (with a number plate) if red were not stop and proceed but restricting. On one of my orientation trips, we were following another train on a yellow, popped around a corner and blew past a red. The engineer went from power to full dynamics and we got slowed down to 20 about a mile later. Had a weed weasel or the FRA been there ... The engineer was one of those conductors that never should have been forced to engine service. Nice guy, I heard he had blown a red signal about a year earlier, and later that he blew through a Form B. Another interesting thing is part of the CTC had searchlights, the rest had color lights. The color lights weren't typical as red was on top. All the sidings were in poor shape at 10 mph, and the railroad only required loaded trains to hold the main. The entrance signal at a siding had four vertical lights in the same head, to enter a siding you had a top red and a bottom as lunar for diverging restricting.

Part of the FRA's justification for forcing railroads to install PTC was for increased capacity, since the deluxe DTC system is supposed to maintain a constant minimum following distance by cab signal versus fixed blocks with wayside signals.

Not sure what they were doing on the Needles sub. Remember that train stopping distances increase dramatically on steep downgrades. I also noticed that when UP recently installed new signals on the old SP in the San Joaquin Valley that they respaced many of the signals, not such if it increased, decreased, or was the result of correcting spacing where sidings were long removed.