If you notice that network performance has reduced significantly when you have changed one or more traffic streams or links in your network from PDM to CTM the most likely reason for this is the fact that CTM models blocking back effects within the network, while the PDM does not.
Another reason can be due to a network coding mistake if you have any unrestricted traffic streams or links in your network. When switching from PDM to CTM the Traffic Stream data item ‘Cell Saturation flow’ is used by TRANSYT 14, and must be set to a value appropriate for the number of lanes. If you forget to set these and have unrestricted traffic streams representing more that one lane, you can be erroneously restricting traffic flow. This restricted traffic also has the potential of blocking back into the network causing considerable blocking effects.
If unsure what value to set for the Cell Sat Flow, a good starting point would be to set the values to what RR67 would give you if all traffic was assumed to be going straight ahead, summed for each lane represented by the traffic stream. You can also take account of the standard RR67 reduction for a kerbside lane. Using this method will ensure that the relatively high initial capacities that are achieved at the upstream end of the downstream traffic streams (due to short headways) are modelled. However, you may wish to use a lower cell saturation flow, that more realistically reflects the overall capacity of the traffic stream.
Traffic stream capacity (as opposed to stopline capacity) is determined by ‘headway’ – which is affected by many factors including, but not limited to, vehicle speed, lane width, road works, parked vehicles, % of heavy vehicles, weather conditions. Taking account of these effects can improve the accuracy of your model. You may find TA 79/99 “Traffic Capacity of Urban Roads” a useful reference.