Network Integration

2.5 Integration with the SIDRA Network Model

The SIDRA Basic Network is integral to ASSIGN both in the configuration of the Assign Network and its use of the SIDRA Network Model. Zones are connected to the Basic Network to form an Assign Network. Movements for each Movement Class (MC) are translated into a movement network for each Movement Class that is used by ASSIGN. Routes are determined either by the user or programmatically as a set of movements that connect the Origin to the Destination. The Network Model is also used to calculate movement travel times and cost. Sites, Routes and Networks are prepared in the Site and Network Tabs. Refer to the SIDRA User Guide for preparing Sites, Routes and Networks.

Processing an Assign Network starts with processing the Network to determine an initial estimate of Movement Travel Times and Cost. ASSIGN then sets the Vehicle Volumes for Sites in the Network according to the selected Traffic Assignment Method. The Network is re-processed with the ASSIGN-determined Vehicle Volumes to update the Movement Travel Time and Cost estimates. Traffic Assignment is then re-applied to adjust the Vehicle Volumes. This process is repeated until a traffic assignment solution is found. Signal timings determined for the Network and used by Assign are calculated as specified in the Network Timing dialog and based on assigned O-D Volumes and Initial Vehicle Volumes.

It is important that Sites use Movement Classes consistently. Routes generated by ASSIGN are formed by connecting enabled movements (specified in the Lane Disciplines dialog) by Movement Class. Hence, Trips between an origin and destination for a Movement Class need to have at least one valid route of interconnected movements for the movement class. A processing error will result if there is no valid Route.

A movement that is enabled for a Movement Class but with zero volume is considered a valid and potentially feasible movement for the Movement Class, hence may form a useable route for the Movement Class. To examine a potential movement travel cost with zero volume, a Minimum Site Movement Demand (default is 1 veh/h) which applies to each Movement Class is set programmatically. The Minimum Site Movement Demand is needed for estimating movement travel time and the Minimum Site Movement Demand set to a low value to minimise its effect on outputs. It is recommended to disable movements for a Movement Class in the Site Lane Geometry dialog where it is considered not practically viable or not likely to be utilised to minimise the effect of the Minimum Site Movement Demand on outputs.

Vehicle Volumes specified for Sites that are normally used for Network Analysis and not used by ASSIGN can either be ignored or used as Initial Vehicle Volumes. Pedestrian Movements and Volumes specified for the Network will always be used by ASSIGN.

If applicable, Routes defined for the Basic Network could be configured as Fixed Routes. The Route must start from the approach that connects to the Origin Zone and ends with the exit that connects with the Destination Zone. The Movement Class for the Fixed Route must be enabled on all movements along the Route in the Site Lane Geometry dialog.