Levels of Detail for Traffic Signal Modelling<br />Jonathan Slason (Beca) <br />Presented: SNUG 16 November 2010. Wellington, NZ<br />The views expressed are those of the author and do not necessarily reflect those of Beca or any other organisation or agency. <br />Further detail behind the issues raised is available by contacting firstname.lastname@example.org<br />
Traffic Modelling<br />Concept<br />Regional travel demands (public and private) for network/project planning;<br />Travel costs for economic evaluation; and<br />Transport demands and effects of specific infrastructure;<br />Traffic flows for design of infrastructure;<br />Transport demands and effects of specific land use development.<br />Operations and Optimisation<br />Accuracy<br />Detail<br />
Source of our Data<br />Regional Models<br /><ul><li>Those forecasts were developed for testing and developing regional policy and strategies and are hence focussed on longer-term (e.g. 30 year) planning horizons.
Most project evaluations require shorter-term forecasts, typically 5-25 years. For example, assessment of environment effects on major infrastructure (such as noise and emissions), require travel forecasts for opening and 10-years post-opening. </li></ul>Analysis Flows<br />Development of design flows from model forecasts is a distinct process that should be recognised in the study process. That process should consider other data (such as counts), additional model output (such as validation data and path analysis) in determining appropriate design flows from model forecasts. <br />Wherever possible, intersection design should be based on surveyed flows, modified based on the change predicted by the models, rather than based directly on raw model forecasts (i.e. Pivoting)<br />
15min variation in flowsInter and Intra Hour Variation<br />AM & PM Peak Hour Factor = 0.88<br />
Traffic Engineer Role<br />The development of design flows for the operational assessment and design of roading facilities should be recognised as a distinct task from travel forecasting. That is, there is a distinct and crucial process involved in taking data from various sources (such as model forecasts and count data), and creating a set of flows suitable for the design process.<br />It should be the responsibility of the person doing the operational assessment and/or design to prepare those design flows, rather than simply adopting outputs from traffic models without consideration of the model type, detail, age and level of local validation.<br />
Traffic Engineer: Flow analysis tasks<br />While design flows for new facilities will generally require input from models, available survey data and data from similar locations should also be considered. In using traffic model outputs to create design flows the following should be considered:<br />The level of network detail in the model. Specifically, if the zone and network detail is low, than forecast turning movements may not be accurate, even if link flows are considered appropriate. Judgements should be made about the precision of turning flows, especially where the modelled flows appear unrealistically low;<br />The age and level of local validation. Additional data or information form the model should be obtained to estimate the likely impact of any material validation discrepancies;<br />The choice of design horizon and design-hour, and how these relate to the modelled years and periods;<br />The time periods being represented, and what peaking factors should be applied to the forecast flows;<br />What upstream/downstream constraints may influence the forecast flows;<br />What level of delay is predicted at the facility being designed, and hence if predicted flows are likely to be influenced by those modelled delays. For example, if the model has high delays at the facility of interest, then the predicted flows could be lower than is likely to occur if the final design has reduced those delays;<br />The sensibility of the flows, when considering existing travel patterns. Various outputs from the models can be requested to gauge the sensibility of the forecasts, including select-link analysis, flow difference plots, delay and/or Volume/Capacity ratio plots etc.<br />As a general rule, survey data should be used wherever practical to validate the forecast flows. Wherever appropriate, local survey data should be used directly, with design flows created by pivoting off those flows using the predicted change in behaviour forecast by the model (i.e. the models are used to predict the change in flows, rather than the absolute design flows).<br />
When to use what Model: Planning<br />Purpose & Need: Enable project designations to be established. Create long-term project price estimates, scale of work, etc.. <br /><ul><li>Focus on Peak hour(s) of operations
Can assume certain sub-groups are operating (tidal flows?)</li></ul>Input: Planning flows into the future (10+ years). Can assume similar seasonal peaking as today. Use PHF of 1.0. <br />Analysis:<br /><ul><li>Only appropriate used is to use deterministic.
Variation or lack of small confidence intervals in inputs suggest little real value in other stochastic or micro models</li></li></ul><li>When to use what Model: Design<br />Purpose & Need: Meet design year performance targets put forth by stakeholders and RCAs. <br /><ul><li>Focus on Peak hour(s) of operations
Can assume certain sub-groups are operating</li></ul>Input: Design Hour flows into the future (~5-10 years). Can assume similar seasonal and intra-hour peak factors as today.<br />Analysis:<br /><ul><li>Opportunity to use deterministic. Optimal to calibrate/validate a similar model before use on new project.
Use of Micro-Deterministic to help with complex junctions and test coordination</li></li></ul><li>When to use what Model: Operations<br />Purpose & Need: Highly accurate flow dynamics to capture vehicle platooning, arrivals, departure, headways, speeds, etc.<br /><ul><li>Focus on Peaks or Specific Periods of Operations with similar characteristics
Can “largely assume” certain sub-groups are operating but needs analysis flexibility to test different arrivals at signals and the intersection response.</li></ul>Input: more certain traffic flows. Current day flows with high degree of confidence<br />Analysis:<br /><ul><li>Limited opportunity to use deterministic. Only if calibrated well
Use of Micro-Deterministic combined with Micro-Stochastic</li></li></ul><li>Other considerations<br />Other Modelling Inputs:<br /><ul><li>Multi-Modal. If Cylist and Ped delay is critical, then specific models may to have be included. Especially pedestrian arrivals from one junction to another. Micro-sim only choice.
Pre-Emption: Micro-models include pre-emption effects
Origin-Destination and Network Flows: micro-deterministic and stochastic programmes
Visuals: often clients or stakeholders prefer visualisation techniques to communicate project </li>