Forecasters rely on different techniques depending on the available information. Growth rates from historic traffic counts, adjusted to AADT, by application of appropriate factors, are derived and checked for reasonableness. The growth rates are then applied to a base year volume and projected forward to the design year. Projections obtained this way represent unconstrained demand for the future.

A constrained forecast is for the final design of a facility, where expected traffic volumes will be limited by the ultimate capacity of the facility. When using constrained forecasts, the future demand is “sized” to the design of the facility and not the “true” traffic demand. For example, if the demand is for a six-lane facility and a four-lane is being designed, this fact should be noted in the Project Traffic Forecasting Report that four lanes will not be adequate for a 20-year design, and necessary steps should be taken to address the potential shortfall.