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Summary
  • Although there is excess capacity on road  links, highways engineers are predicting problems at junctions, including the new £1m junction at Sycamore cross.
  • The signalised rotary at Culverhouse Cross (it's not really a roundabout) is already operating above its capacity, and predictions for 2026 show that this situation is going to get worse, with queues back along the A48 longer than the link.
  • There is no data concerning capacity of the existing junctions in St Nicholas
  • There is no mention of latent demand at the already congested junctions 

The following information is from the Highway Impact Assessment.

Click map for larger image:




The Capacity of Road Links




The Capacity of Junctions

JUNCTION 4 – A48/FIVE MILE LANE/ROAD TO PENDOYLAN JUNCTION 
 
Junction  4  is  a  staggered  4  arm  traffic  signal  controlled  junction  with  controlled pedestrian  crossing  facilities  on  each  approach.  The  pedestrian  crossings  are controlled by an all red to traffic stage. 
 
It is unknown how often the pedestrian crossing facilities at the junction are called during the AM and PM  Peak Periods, therefore, analysis of the junction has been undertaken  with  and  without  the  pedestrian  crossings  appearing,  providing  a  best and worst case scenario. 
 
No  existing  queue  information  has  been  provided  in  order  to  validate  the  existing predicted modelled queues against.  
 
Saturation flows have been calculated based on geometry and the results optimised for delay. 
 
The  junction  has  been  analysed  using  JCT  Consultancy’s  software  programme LINSIG version 2 and the results summarised in the following tables below; 
 


In  2012  AM  and  PM  Peak  Periods  the  junction  operates  within  capacity  with  only  traffic stages appearing but over capacity if the pedestrian stage appears every cycle.  
 
In 2026 AM and PM Peak Periods the junction is predicted to be over capacity with or without the pedestrian stage appearing every cycle. 

JUNCTION 5 – A4232/COWBRIDGE ROAD/W/A4050/A48 ROUNDABOUT 
 
Junction 5 is a traffic signal controlled roundabout. 
 
No  existing  queue  information  has  been  provided  in  order  to  validate  the  existing predicted modelled queues against.  
 
Saturation flows have been calculated based on geometry and the results optimised for delay. 
 
The  junction  has  been  analysed  using  TRL’s  software  programme  TRANSYT version 12 and the results summarised in the following table below; 


Cardiff  City  Council  have  stated  the  existing  roundabout  operates  under  a  128 seconds  cycle  time.  The  roundabout  nodes  are  double  cycled  and  the  exit pedestrian crossings are single cycled. The analysis has used the same cycle times and double and single cycling.  
 
In 2012 and 2026 AM and PM Peak Periods the junction is over capacity. 




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