A critical review of Austroads Report AP-R621-20
Revised version published as Austroads Report AP-R647-21 “Management of Traffic Modelling Processes and Applications”
Peak demand flows
The SIDRA user guide and the supporting research publications make it clear that demand volumes for peaking are modelled either (i) by specifying hourly (or longer period) demand volumes but allowing for peaking within the hour by specifying a Peak Flow Factor, PFF, or (ii) by specifying the demand volumes for a shorter peak period if the volume counts are available. In either case, the analysis is done for the peak flow period.
SIDRA uses default peak flow periods of 30 minutes for the Standard Left software setup (applicable to Australia and New Zealand) or 15 minutes for the HCM software setup (applicable to US as specified in the Highway Capacity Manual).
Single or multiple flow periods?
Akçelik, R. and Rouphail, N.M. (1993). Estimation of delays at traffic signals for variable demand conditions. Transportation Research 27B (2), pp 109-131. View Publication
Analytical models are capable of modelling congestion affects with the analysis of a single peak flow period. On the other hand, microsimulation using individual vehicles needs to use multiple flow period modelling. The reason is explained below.
For oversaturated conditions, the SIDRA delay is the average delay to vehicles arriving during a given flow period (analysis period) including the delay experienced after the end of the flow period until the departure of the last vehicle arriving during the flow period (which happens after the flow period). This corresponds to the path-trace (instrumented car) survey method of measuring delays. This differs from the method which measures the delay experienced by vehicles in the queue during the analysis period only which corresponds to the queue-sampling survey method.
In a microsimulation model, delay experienced by vehicles that still remain in the queue at the end of the flow (analysis) period can only be measured by simulating an additional flow period. On the other hand, analytical model is able to determine this delay without processing an additional flow period. The figure given here shows a deterministic oversaturation model chosen as a simple model to explain the measuring of delays experienced by vehicles in oversaturated conditions. For more details on this subject and further discussion on time-dependence of demand flows and variable demand analysis, refer to the review document.