How can we connect network with zones in Visum - review of options + i2 connector balancing method

Hi, 

I'd like to report here on what options do we have to connect Visum network. There are numerous ways available in Visum, and they can significantly alter modeling results.

 Now let me describe you the basic options provided, and the script written by i2, further extending basic capabilities. 

Basic info on how to model connectors can be found in Visum Manual, and the scheme of available options is shown below (fromVisum manual):

  See the results for various options below: 

1. Default 

if we do not change default setting, the traffic will start path searching at the zone centroid, and will use shortest connectors in shortest path search.

As we can see on the sample below, inner links of the network are empty, due to the fact that all of assigned traffic used connectors, which are optimal in shortest path search during assignment. This will happen if our connectors are “faster” than links. Also, traffic volume of single OD pair in most cases will use just one connector, which can be unwanted outcome.
When we use this method with multiple connectors we will most probably struggle to balance the network to reflect actual traffic from the zone.  Personally, It's hard for me to tune connectors' impedance in Visum (You don't have capacity as for links, you can only tune travel times)

Also, such a model will probably cause no significant harm in dense, urban model, where zones are relatively small, but imagine that your zone is a region in regional model, where zones are much bigger. The model below will drastically bias traffic resulting to underestimating inner traffic, etc.

Also one can notice that trip destination distribution at connector heads are different for each node (head).

fig. 1 Network with 5 zones -one in center,and foru at the sides. Red color is for links, and blue color is for connectors. 

2.      Regard connectors as shares – Standard

This option tries to balance Volume of each connector, regardless OD of traffic on it. Visum achieves it via “dummy” capacity restrain where impedance is calculated according to virtual capacity. Unfortunately we cannot save those impedances, so the connectors volumes can be different when we slighlty change network (see the last paragraph).   

We can see that all connectors are used by traffic, and volumes are comparable, however results are various (Vol min: 319,  Vol max: 526). Probably we could obtain more balanced result if we used higher travel times for connectors (to influence path search results).

With this option, distribution of trip destination at connector heads is still different for each node, but the results are slightly more balanced.  

fig. 2 Network with standard Visum connector balancing procedure (weights = 1)

3.       Regard connectors as shares – Each OD Pair

This option lets us obtain balanced assignment, where each connector has exact portion of traffic calculated due to its weight (here all connectors have weights equal to 1).

Additionally, at each connector ending point we have the same trip distribution, so that executing flow bundle will give similar results for each connector.

It's my favourite choice, andgives expectional results, however we need to remeber, than it significantlyincreases calculation time, as each connectors is regarded as a new virtual zone. So if we have network of 500 zones, and each zone has 10 connectors, than this option will increase dimension of calculation matrices from 500x500 to 5000x5000 , so that the thing to remember! Also it can exceed Visum licence for zones and outcoming error is yet unhandled as far as I'm concerned.

fig. 3 Network with Visum connector balancing procedure for each OD pair (weights = 1)

4.      i2 balance

This is result of script I have written to balance connectors in the network.

It gives similar results to standard “regard connectors as shares” with capacity restrained connectors, and it has similar logic of balancing.  However, I’ve written it because:

a)  now I’m be able to save connector travel times giving balanced results. In Visum the impedance of connectors is not stored in the network, it’s simply saved as assignment result, so when you want to test new scenario, assignment of the traffic to the connectors can be different. Now I wanted it to be constant and fixed for basic scenario, and that’s how I achieved it (for more info contact me at info@intelligent-infrastructure.eu).

b)  I didn't want to use regard connectors as share, as it increased my calculation time significantly.

c)  I wanted to test quick convergence methods, and statistic based regularity measures. 

fig. 4. Results of procedure developed by i2 tobalance connectors - results are similar to fig. 2 however can be stored to keep assignment results.