Headway Calcs- choosing 3 or 4 aspect signalling

[Peter]

Hi All,

I'm planning to sit for module 2 this year.
i'm little bit confused with choosing of Aspect.
Can any one please justify why we need to choose 2 Aspect if N>3.66 and
3 Aspect if 3.66>N>2.66..

very thankful in advance..

Regards
Arjun.

By asking the question, you have illustrated the dangers of learning and quoting a formula without understanding it and it is a credit to you that you are asking. My suggestion to you would be to work back and draw out some examples of what a layout would look like with signal spacing relative to braking distance for a range of numbers for N and you will see that the oft quoted "magic" numbers relate to the boundaries of the requirements (spacing greater than min braking, but not giving excessive braking). At a given point, 3 aspect will start to give you excessive overbraking so you have to go to 2 aspect. The precise breakpoint will of course depend on the tolerance to overbraking that your railway practice has.

Peter