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A block section must be long enough that a driver seeing a caution aspect can stop before the next signal: braking distance plus sighting distance plus the overlap beyond the signal, with a safety margin. That length then sets the minimum achievable headway on the line.
Signal spacing
spacing = (braking distance + sighting distance + overlap) x (1 + margin)
Rail results are planning estimates only. Verify against the infrastructure manager's rule book, approved braking curves and applicable railway safety regulations.
It protects against a slight overrun past a red signal, keeping the conflict point clear. Typical overlaps are 180-200 m.
Shorter blocks allow closer following and higher capacity, which is why resignalling with more, shorter blocks or moving-block ETCS Level 3 increases throughput.