Trip generation is calculated for both variants of the standard 4-step model in a separate procedure (Standard 4-step model in two variants). In that stage for each zone and each demand stratum the production and attraction rates are calculated. These parameters are also called productions and attractions. The productions either correspond to the actual origin traffic of the zone i.e. the number of trips starting there, or the attractiveness of the zone for the demand stratum, meaning they have an influence on the probability of trips starting in that zone with the next Trip distribution procedure. Which of the two cases applies can be determined by a procedure parameter of Trip distribution. The same holds for destination traffic.
The productions of a demand stratum in a zone depend on its structural or demographical indicators describing the intensity of the production activity. For the production activity “Home” the number of inhabitants of a zone, if necessary, disaggregated into age, income and/or car availability can be used. For the production activity “Work” the number of jobs may be appropriate, perhaps broken down into different sectors. For such skims user-defined zone attributes are the best. First, production Qi of zone i is calculated with the help of a formula,
whereby SGg is summed up across all structural properties. SGg(i) designates the value of SGg in zone i. The coefficient αg is a production rate which describes how many trips per structural property unit occur. They specify the production rates per demand stratum and zone attribute used. The same calculation is performed for the attraction Zj.
In most applications the total production of a demand stratum (added up over all zones) corresponds to the total attraction.
If equality has not already been the outcome of the attributes and production rates used, it can be set by means of a procedure parameter whether all productions and attractions have to be scaled so that their totals are equal. As reference values you can predetermine total productions, total attractions or the minimum, maximum or mean value of both parameters.
You can limit calculation to the active zones. This might be useful in cases where the network model covers both the actual planning area and its surrounding sub network cordon zones. If you only want to calculate planning area-internal trips by means of the demand model, first of all define a filter for the zones of the planning area only. Proceed in a similar way if the production rates are not uniform for all zones. Break the zones down into groups of homogeneous production rates and insert the operation trip generation for each of the groups into the process. Prior to each such operation set a filter for the zones of that group (operation Read filter (Using Visum: Reading filters)) and calculate trip generation only for the respective active zones.
For each zone the results of trip generation are stored per demand stratum in the zone attributes productions and attractions.
