Theoretical and Empirical Relationships for the Quality of Flow and for a New Level of Service on Two-Lane Highways

This study has three main objectives: (1) to develop new, theory-based, queuing relationships for the quality of flow on two-lane rural highways; (2) to estimate the relationships both from the empirical data that were collected on 15 two-lane rural highways and from the queuing models that were developed; and (3) to propose a new level-of-service variable that measures the quality of the flow both inside and between platoons. The paper presents five flow-characteristic measures for two-lane rural highways: the flow, the average platoon length, the traffic intensity, the percent-time-spent following, and the freedom of flow. It is shown that the five measures can be calculated from easily collectible data parameters and also from empirical models related to the two-way flow that are developed based on the field-data collected. It is proposed that the level of service be estimated from the freedom-of-flow parameter eta, which is developed in the paper. The relationship between eta and the two-way flow is calibrated from traffic observations. Level-of-service thresholds, based on the flow and on eta are presented and discussed.