Impact of Free Flow Speed Estimation Method on Project Prioritization

Transportation project prioritization relies heavily on assessments of traffic conditions and performance metrics; however, inconsistencies in analytical techniques can significantly skew the results. This study conducts a comprehensive examination of free-flow speed (FFS) estimation methods and the impact of delay threshold selection on delay calculations, demonstrating their pivotal roles in result interpretation and project prioritization. A comparative analysis of several FFS estimation methodologies reveals that methods based on the 85^th percentile of observed speeds yield FFS estimates that are highly correlated. In contrast, the methods based on the 95^th percentile observed speeds or based on simple arithmetic operations of posted speed limits generate FFS values that diverge notably from the 85^th percentile techniques. Moreover, the study underscores the substantial influence of delay threshold selection on the calculated delay, exhibiting significant variation at statewide, district, and corridor levels. The findings indicate that the choice of threshold not only influences the absolute delay magnitude, but can also shift perceptions regarding the severity of congestion in different regions or corridors. The research results underscore the vital need for transportation agencies to adopt standardized methodologies for FFS estimation and congestion delay threshold selection in order to enable consistent, unbiased project evaluation and data-driven prioritization.