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Forecasting modelling myths

Forecasting is sometimes faced with criticisms which are based on common misunderstandings about travel demand forecasting. Some of the more persistent myths are presented here along with explanations of the facts.

  1. "Because the model is based on surveys of actual behaviour, it can only predict trends and not changes in behaviour."
  2. "The assumptions contained in the RTM cause sprawl."
  3. "The RTM is the Scenario Series and vice versa."
  4. "The model said the road is needed (or, not needed)."
  5. "Every time we use the model, it gives us bad news."
  6. "Transit mode share is a model input."
  7. "The RTM is a traffic model."
  8. "The model takes too long to run." 

1. Myth: "Because the model is based on surveys of actual behaviour, it can only predict trends and not changes in behaviour."

Explanation: The RTM, like all models, is designed to test the effects of changes in a variety of parameters including behaviour. While it is typical to create one future scenario to reflect current trends and behaviours, alternative scenarios can be created to analyze departures from those trends.

2. Myth: "The assumptions contained in the RTM cause sprawl."

Explanation: This statement confuses the RTM with its use in the decision making process. Assumptions are made in order to represent a future condition. Decisions using model results should be made with a complete understanding of the underlying assumptions. Further, these assumptions can be changed to test alternative scenarios and, in fact, comparative analysis remains one of the strengths of modelling.

3. Myth: "The RTM is the Scenario Series."

Explanation: The RTM is the model framework in which scenarios are tested. The RTM is like a calculator. Data are entered in the form of a formula and a result is generated. The RTM works much the same way. Assumptions are entered in the form of a scenario and results are generated. The Scenario Series, on the other hand, is a set of successive time horizons which attempt to reflect the city's current policies around land use and transportation. The series is used a baseline that alternatives can be tested against and thus aid in decision making. The current Scenario Series is based on the recently approved CTP/MDP.

4. Myth: "The model said the road is needed (or, not needed)."

Explanation: This statement is often used by those who do not want to take responsibility for difficult decisions. Results from a scenario require interpretation, analysis and judgment. The model is merely a calculator, albeit complex, that is used to show the impacts of land use and transportation interactions. Expectations that the model is some sort of 'oracle' that will reveal the future or make decisions are completely false.

5. Myth: "Every time we use the model, it gives us bad news."

Explanation: Similar to the statement above, this criticism is used by those who have already made up their minds about the outcome of a scenario before it has been tested. The model is used for evidence based decision-making.

6. Myth: "Transit mode share is a model input."

Explanation: This has not been true since the mid 90s when mode choice was not included in the model framework. The current model form estimates the choice to use any mode - including transit – based on a variety of factors, the most important being the overall cost of the trip. In the model validation phase, mode share results from the base year are compared against field observations of transit use.

7. Myth: "The RTM is a traffic model."

Explanation: The model structure explicitly includes the transit, walk, bike, and commercial vehicle modes in addition to the auto mode. The mode choice behaviour is obtained from surveys. Information related to each of these modes is available for analysis.

8. Myth: "The model takes too long to run."

Explanation: Model run times range from a few minutes to a few days depending on the type of scenarios being tested. What takes time is the conception of the scenarios, the preparation of the data representing the scenarios, and the analysis and production of the results. Scenario conception, which is the responsibility of those requesting the analysis, can be quite time consuming if the concept lacks clarity and detail. Data preparation and scenario analysis are entirely dependent on the complexity of the scenarios. Managing the dependencies and resources involved in scenario analysis is an important part of good project management.

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