Back to Visual-tm home Back to overview Visual-tm has a more versatile way of modelling the day which enables (if you wish) the whole day to be modelled as a continuum. This covers both private and public transport and adds a whole new set of tools for the modeller to use. (He can also use the conventional methodologies if he wishes). The day can be divided into consecutive time periods and assigned in sequence with the output from one time period, as the starting condition for the next. It can be used for both highway and public transport assignment.
Trips in transit at the end of one time period are carried over into the next, with each part put into its correct assignment. So the assignment model only assigns trips to links if the trips are on the links within the time period being modelled. Those links which the trip goes along after the end of the time period are assigned in the nest time period. This is achieved by the assignment producing a too-late trip matrix which consists of all the trips which did not make it to the final destination within the time period being modelled. These are given by origin-destination zone pair and gives the number of trips, the time of departure from the origin and the node reached. This can be input to the assignment of the next time period which finds the path they would take and loads them onto the network for that portion of their path falling within the time period being modelled.
The starting queue at the beginning of the time period being modelled can either be estimated from counts or by running the previous time period in the assignment model. The day’s consecutive time periods could for example be midnight, 6.00am, 8.00am, 9.00am, 12.00 noon, 2pm, 4.30pm, 5.30pm, 7.30pm, midnight. Each one could, if necessary, have an assignment model so as to trace the queue build-up and decline throughout the day. Conventional models of the morning peak, evening peak hour and inter peak are entirely separate but with Visual-tm they can be connected together.
To get this greater precision into the assignment model so as to better model effect of time-dependent queuing into our junction modelling required a new approach to modelling time (in the sense of time of day) which we have called clocktime. The path builder now has the option of recording the clocktime the path gets to each node, or boards or alights each public transport service in the network. For highway assignment, this is called clocktime assignment and for public transport it is called timetable assignment.
Time periods are generally for one to two hours and represents the time period covered by the trips in the assignment trip matrix. Time periods are divided into ten-minute time intervals and the clocktime/ timetable assignment keeps track of the clock time at each network node. Junction and interchange turning movement flows are accumulated for each time interval and passed to the junction and interchange simulators which pass back turning delays in each time interval, for the next assignment iteration.
Time intervals are divided into one-minute time slices. The trip matrix, which represents one time period’s worth of traffic, is divided into one time slice’s worth of traffic within the assignment process. Time slices are accumulated into time intervals for the highway junction and public transport interchange simulation. Time slices are also accumulated and output to the too-late trip matrix for assignment in the next time period.
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