From: rcp27g@gmail.com   
      
   On 2015-02-13 20:23:01 +0000, Marc Van Dyck said:   
      
   > rcp27g@gmail.com explained on 13-02-15 :   
   >   
   >> or putting in positive singalling control on others (ie where the   
   >> crossing is protected by railway signals that aren't cleared for the   
   >> train until the barriers are down and the crossing positively checked   
   >> to be clear).   
   >   
   > This is perfectly feasable but requires to order the gates to go down   
   > at a distance that is longer than the braking distance of the train.   
   > This means there will be a long delay between the gate going down and   
   > the train actually passing the grade crossing.   
      
   Indeed, this is the case.  It is less convenient but allows for   
   positive safety.   
      
   > As it has been mentioned,   
   > motorists are unpatient creatures; if the delay is too long, people   
   > think the gates are faulty and start turning around them. You might end   
   > up this way with a grade crossing that is inherently less safe, because   
   > of human nature...   
      
   Solved by making the barriers block the whole road.  As the crossing is   
   positively checked to ensure the barriers are down and the crossing is   
   clear before clearing the signals for the trains, the issue of cars   
   being trapped within the crossing is avoided.   
      
   The usual design in the UK, for example, is for the road to be blocked   
   by four barriers: each covering half the roadway on each side of the   
   railway.  The warning lights/klaxon start first, then the "entry" side   
   barriers come down, then the "exit" side barriers come down, then the   
   klaxon stops (it gets irritating, and once the barriers are down, is   
   redundant).  Then the crossing is checked to be clear, either by a   
   person in a local signal box or by CCTV cameras, and if it is clear,   
   the signals are cleared for the train or trains passing.  It is safe,   
   but means waits at the crossing can be a couple of minutes before the   
   train actually arrives.  The crossing sequence is initiated by the   
   signalman, who can keep the crossing down if there are multiple trains.   
    They can be annoying, but they are safe.   
      
   Not all crossings in the UK are of this sort, there are also "automatic   
   half-barrier" crossings that do not block the whole road and are   
   triggered automatically by the train approaching, without positive   
   safety, but designed for the minimum road-closed time, so the barriers   
   come down about 20s before the train arrives.  Such a crossing was   
   involved in the Ufton Nervet crash, and they are only used on quiet   
   roads with little traffic, and on railway lines with lower speeds.   
      
   Robin   
      
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