Signal Spacing and headway calculation

[Peter]

Hi Signalling Professional:

I have questions regarding the calculation for signal spacing and headway and I would like to correct myself if I am wrong.

Situation:
Line speed is 100km/h, max train speed is 120km/h, required headway 3min

My question is which speed of the above do I use for calculating the signal spacing and headway.

My understanding is that:
a) when calculating the signal spacing use line speed 100km/h (that's the speed allowed to run the train normally)

b) when calculating what signalling system to be used for the railway system to suit the required headway uses the train speed (120km/h) that's the speed the train could possibly run on the line.

Maybe this is simple question, but am I right for this ?

thanks

No, exactly the wrong way round!

The minimum signal spacing (equivalent to braking distance for three aspect) is governed by the distance a train needs to stop when it is travelling as fast as it can - max linespeed.

The speed that is used for validation of the headway is usually taken as a speed lower than the max linespeed since not all trains will always be travelling at linespeed at all times. If the train out performs that (ie does manage to travel at linespeed), then it is out of the way quicker so will not impact on the following train.