I am trying to compute the modal shift in a new public transit scenario. This is being done by first taking the full network in the first case and then omitting a new public transit line in the second case. What is the exact way to calculate the mode shift on new transit line scenario.

# Modal Shift Computation

Your question is not really clear. Are these two cases separate scenarios? Are you also modelling highway trips as well and do these share the same demand input process as the public transport? What do you mean by error in model shift calculation?

Thanks for your reply james. I just want to find the exact share of passengers shifted from different modes when new mode introduced in the model. I tried to solve this by just finding difference of ridership of different modes before and after adding new mode. But this way is not performing well.