Well railway signal visibility in the UK is required to be over 2 miles by law – in that a driver must be able to distinguish the aspect of the signal from that distance. As defined by the RSSB, Regulation GE/RT8037. Particularly on high speed sections where the limit of speed means a train is travelling a mile every 30 seconds.
Braking Distances are variable based on train weight vs speed vs grade however the legal requirement for braking from 125 to 0 in the UK is 2050m on a level grade and 3500m on a falling grade of 1:100. See RSSB Regulation GK/RT0075.
Should you introduce this requirement then you would need to position signals at appropriate distances apart and you’d need some way to measure it. In the UK particularly on high speed sections, they’re placed at intervals of 2km unless the grade of the line and the inclusion of diverging routes require an extended or decreased area of protection particularly for heavier trains. You would also need some means to measure the grade of the track so that you can adjust for placement of related signals.
By pre-signal I assume you mean a Distant and/or Outer-Distant? Also referred to as repeaters or related signals.
The system you say is simple in the UK is actually infinitely more complex than you think it is. Particularly when it comes to signal control types as well as signals with route indicators and limited aspect displays as well as approach control and interlocking. Throw on top of that ERTMS and TVM430, ATP, TPWS, AWS and RETB and it gets even more complicated.
