Variability in Traffic Flow Quality Experienced by Drivers

Abstract

Freeway operating conditions are typically evaluated on the basis of the level-of-service (LOS) concept, which is defined according to traffic density, a macroscopic traffic parameter. Although density may provide general estimates of current traffic conditions, it generally fails to communicate the variability in the quality of traffic flow experienced by individual drivers. This variability may be caused by factors that are not effectively captured by density, such as travel lane, vehicle position within a platoon, and characteristics of roadway geometry. This paper investigates the variability of traffic flow quality as indicated by measurements of speed and acceleration noise (standard deviation of acceleration) on the basis of field data obtained from instrumented vehicles equipped with Global Positioning System devices. The microscopic measures observed from individual vehicles are compared with density-based LOS, which are calculated with data from the Atlanta, Georgia, traffic-monitoring system. The comparison indicates that under the same LOS condition, drivers may experience significant differences in the quality of traffic flow. In addition, the comparison indicates that drivers under different density-based LOS conditions may experience a similar level of traffic flow quality. The variability generally becomes larger as traffic conditions worsen.