This section provides a description of appropriate methods and examples for forecasting future traffic in areas without a travel demand model and provides a basis of comparison to model forecasts in areas with a model.

For areas without a travel demand model, forecasting is normally based on historical trends. Growth rates may be developed utilizing US Census data, employment data, and by working with the relevant county, city, and other local government agencies and using information from their comprehensive plans. When historical AADT data is used, a regression analysis is performed using the most recent ten years of data, when available. Even though linear growth pattern is normally assumed, care should be taken to examine the growth trend in the past and any constraints or policy changes that may alter the development pattern in the future. Other forms of growth patterns such as exponential, decaying exponential, or composite growth patterns can also be used for analysis. The historical data need to be reviewed to check for consistency and reasonableness. Outliers should be reviewed and removed from the analysis if no logical reason(s) could be found for the inconsistencies.