Modelling tools in the SUMP process
A transport model is a simplified representation of the real world that allows for testing and evaluating theoretical (“what-if?”) scenarios. The role of transport models is to support public authorities in the design process of future transport infrastructure (including new pedestrian and cycling facilities), and new or changed operational concepts (e.g. intelligent signal control systems). They are continuously developed and adjusted to current mobility trends, sociodemographic changes and sustainable environmental objectives. Typical results include total travel time for the different transport modes and user groups, volumes in the private and public transport networks, emitted air pollutants, etc. The outputs of transport models thereby often feed into further economic calculations or are the basis for political discussion including public involvement.
A transport model can be used to generate reliable and consistent input to the SUMP process, specifically in certain planning stages such as scenario development, measure appraisal and selection, and monitoring. Modelling results help to predict the impact of different combinations of policies and measures, taking into account the complex interactions and potential reinforcing or rebound effects, thereby helping to define the most effective integrated packages. Beyond their use to define the baseline scenario, they also enable regular monitoring of changes in the transport system during the implementation phase to assess whether you are on track or if you need to react and adapt your actions.
The decision about whether or not to use transport models for the SUMP needs to be taken early in the SUMP process. This depends on the time horizon of the SUMP as well as on the nature of measures under discussion: the more it is expected that these measures will impact transport demand (such as the construction of a new public transport line, the introduction of a new sustainable mode or service, etc.), the more it is recommended to use transport modelling to predict these impacts. The available budget, time, data and the scale of questions determines which model is used.
The three categories of transport models are macroscopic, microscopic and mesoscopic, with the first two being the most commonly used. Macroscopic modelling is typically applied for strategic planning, whereas microscopic modelling is typically applied for operational planning. Macroscopic models focus on large-area choices such as destination, mode and route choice, while microscopic simulations mostly focus on the traffic flow model. Thus, the appropriate modelling level has to be selected to analyse the various impacts of the cities’ measures, as they may differ according to their scope.
Up until recent years, the available modelling tools have not fully considered cycling and walking. The EU-funded CIVITAS FLOW project (http://h2020-flow.eu) worked to improve micro- and macroscopic transport modelling software so that they can more accurately model the existing cycling and walking infrastructure, as well as cycling and walking behaviour. The developments include the extension of the macroscopic travel demand model (including introducing vehicle sharing systems and enhanced stochastic assignment for cycle route choice) and the improvement of features of microscopic transport simulation software (including improved modelling of the interaction between vehicles and pedestrians).
Another type of model are integrated transport and land-use models (Land Use Transport Interaction - LUTI), which have the capability to simulate a wide range of interventions ranging from infrastructural projects, pricing, regulation, co-modality to planning of urban space. They can also include the impacts of “rebound” effects due to relocations or newly generated demand. However, it is important to highlight that integrated transport and land-use models are complex and data-hungry: their setup requires a significant amount of time and effort as well as technical expertise.
It is important to be aware of the limitations of models at all points in the planning process. Planners and modellers have to use their own judgment as transport modelling isn‘t an exact science and all models have systematic biases. Each model run is based on many assumptions and calculations and each one of them increases uncertainty. Uncertainty is difficult to understand especially when exact figures are presented on a nicely-designed map. There is also a strong temptation to believe forecasts even when they go beyond the capabilities of the model. Uncertainty also grows the more you zoom in. To this end, it is essential to calibrate your model to your local context and not to simply use the default settings.
Therefore, planners’ responsibilities throughout the process are:
To commission a sensitivity test
To present the limitations together with results
To use ranges of outputs and qualitative outputs, not point estimates
To avoid zooming in beyond a credible level
Aggregated models called sketch planning models are no transport models in the sense described above but they could be an interesting option for initial policy screening within the SUMP process. They can be built with significantly fewer resources and allow users to explore and identify appropriate sustainable transport policy measures, quantifying their impacts within a consistent framework and setting up the implementation pathway of future scenarios. One typical example of this category is the Urban Roadmap 2030 model www.urban-transport-roadmaps.eu developed on behalf of DG MOVE). However, aggregated models cannot replace the use of more disaggregated models for detailed assessment.
Author: TRT Trasporti e Territorio, Rupprecht Consult