Two-lane highways: guidance for estimating speed-flow relationships and free-flow speeds from field data

Speed, as for other roadway facilities, is an important performance measure used in evaluating the quality of operation on two-lane highways. The foundation for the use of speed in these analyses is typically a speed versus flow rate relationship, as well as free-flow speed. However, the current Highway Capacity Manual (HCM) lacks specific guidance on developing a speed-flow relationship and estimating free-flow speed based on field data. This paper addresses these gaps by providing comprehensive guidelines for both aspects. An extensive amount of field data from several states were used in this study. For developing speed-flow relationships, this study offers detailed guidelines on appropriate data aggregation intervals and methods for filtering data points to ensure accuracy. For estimating free-flow speed, this study establishes criteria for identifying free-flow vehicles and recommends a range of minimum sample sizes of free-flow speeds observed in the field. Additionally, it provides equations to estimate the time required to collect the desired sample size of free-flow vehicles, ensuring efficient and accurate data collection. These guidelines will enable transportation engineers and researchers to accurately and efficiently develop speed-flow relationships and estimate free-flow speeds based on the necessary minimum sample size of free-flow vehicle observations on two-lane highways.