Assessing quality of progression with cyclic coordination functions

Quality of progression plays an important role in assessing the performance of urban streets with signalized intersections. The Highway Capacity Manual uses progression adjustment factors that modulate control delay at a signalized intersection approach in accordance with the quality of coordination. These factors correspond to six discrete arrival types that need to be estimated from given or observed flow data. However, this procedure is inaccurate and may lead to incorrect determination of level of service. This paper introduces an improved procedure for the determination of quality of progression that is sensitive to different traffic conditions, that can be easily calibrated, and that produces more accurate results. A periodic, continuously variable cyclic coordination function and associated coordination adjustment factors are developed to measure delay or travel time as a function of offsets along an urban street segment. Those measures depend on a variety of factors, including traffic flow characteristics, link physical characteristics, and traffic signal controls. Because quality of progression is periodic with the cycle time it can be modeled as a Fourier series consisting of a sum of harmonics. A few harmonics provide good approximations to the original functions. The study shows how to obtain the principal harmonics from the underlying traffic, link, and signal data, which result in a simple and more accurate procedure for quality of progression estimation.