# Discussion of travel delay in acceptable models in project-level traffic forecasting

Many travel models in the US calculate delays with a “volume-delay function” (VDF) where travel time on a link is a function of the volume on that same link. An example of a VDF is the well-known “BPR curve”. A much preferred method of calculating delays is to use operational analysis procedures from the “2010 US Highway Capacity Manual” or traffic network microsimulation models. Operational analysis procedures incorporate intersection geometry, signal timing and the effects of opposing and conflicting traffic. If a travel model calculates delays with a VDF, exclusively, then the estimated travel times (or speeds) must be viewed with suspicion, even when the model has been intensively calibrated and can show strong validation statistics. In addition, estimated turning movements will tend to have larger errors when delays are calculated with a VDF. Refinement steps are required when using speed and turning movement results for a project-level forecast from a travel model that depends upon VDFs for travel time estimation.