Impedances at nodes

Intersections are modeled as nodes or as main nodes in Visum. Intersections of roads and/or railway tracks are bottlenecks in an urban transport network. At the intersections, conflict points have to be passed in succession by the non-compatible traffic flows. The order in which the flows traverse the conflicting areas depends on the type of control:

To choose the route within an assignment procedure, the impedance on alternative routes is decisive, which results in the sum of impedances of all traversed network objects. The bottleneck effect of a node is thus displayed for all variants of the traffic control by the impedance of the turn used. The impedance of turns usually corresponds exactly to the travel time tCur, thus the time required to traverse the node in the turning direction of the route.

For calculating tCur per turn Visum offers three different models that represent the different compromises between data entry and computing time on the one hand and accuracy and real-life situations on the other.

• Turns VDF (Node impedances of turns VD function)
• Nodes VDF(Node impedance of nodes VD function)
• Intersection Capacity Analysis ICA (Intersection Capacity Analysis according to the Highway Capacity Manual (ICA))

To use ICA during assignment, select the method Node impedance calculation (ICA).

Comparing advantages and disadvantages in Table 96 is to help you choose the appropriate calculation model for your project.

Model	Advantage	Disadvantage
Turns VDF(Node impedances of turns VD function)	Little input complexity (per turn merely capacity and t0) Calculation fast Assignment fast convergence	Time required for the turning movement only takes the turning volume into account, not the amount of possible conflicting volumes (separable cost functions)
Nodes VDF(Node impedance of nodes VD function)	Input complexity only slightly larger than for turn VD functions (additionally capacity and t0 for the node itself as well as designating subordinated links) Calculation fast For subordinate turns at two-way stop nodes, the time required due to its own volume increases by an additional penalty, which depends on the total volume/capacity ratio of the node and therefore on the volumes of conflict flows.	Assignment convergence slower due to the inseparable penalty Compared to ICA, taking conflicting volumes into account is extremely simpler due to the fixed penalty
Node impedance calculation (ICA)(Intersection Capacity Analysis according to the Highway Capacity Manual (ICA))	Impedance calculation precisely considers lane allocation and signal control. Special turn pockets for example, are capacity-increasing and dependent on the entered signal timing, protected and permitted turns are calculated correctly	Input complexity considerably higher: Instead of capacity and t0, model the lane allocation at the node and - where available - the signal control in detail Calculation more time consuming Assignment convergence slow due to the inseparable impedance model, sometimes without additional measures not at all

Table 96: Advantages and disadvantages of the node impedance model

Due to the reasons mentioned we recommend the following for the selection.

• For comprehensive models, modeling with VD functions for turns or nodes is appropriate. ICA cannot be recommended here, because the input complexity for the detailed supply of nodes with geometry and control data is usually too high. Furthermore, the result after each acceptable computing time due to the slow convergence of the assignment still contains approximation errors, which are around the same size as the accuracy gained through ICA.
• ICA however, is the method of choice if you want to subsequently calculate and analyze the performance of one or more nodes of an existing assignment result. This is how you can determine which aspects of the node contribute to a high impedance. It is therefore sufficient to only model those nodes completely which have to be analyzed.

With a classical assignment method (Equilibrium or Equilibrium_Lohse, for example), ICA is only conditionally recommended due to the known convergence difficulties, and it should only be applied to small-scale analyses with some 100 nodes. To avoid these problems, the Assignment with ICA method is recommended.

In most cases you will globally decide on a calculation model. You can however also combine different calculation methods within a network, (for example, Turns VD functions as standard model and ICA simply for very important nodes with complex lane allocation or large conflicting flows).

All calculation models are based on turn volumes in car units per hour, which are determined through the user's settings, either from the assigned volume or from counted data via a factor.