The highway capacity manual (HCM) offers a new model for assessing the effect of signal progression on delay at a signalised intersection. This paper discusses a comparison between the HCM progression model and the progression model used in TRANSYT, a signal design and evaluation program which has been in use for several years. The TRANSYT-7F program is used to compare the delays estimated for various qualities of progression with the delays estimated for random arrivals. Comparisons are made on a single pair of links under controlled conditions and on a network of 85 links under simulated field conditions. It was demonstrated that the two techniques agree quite closely. It was also observed that the platoon ratio R, as defined in the HCM, provides a better predictor of progression quality with heavy traffic volumes. The TRANSYT results suggest that a wider range of progression adjustment factors exist than the HCM recognises, and that some extrapolation of the HCM values may be warranted to cover exceptionally good and exceptionally poor progression. An independent indicator of progression quality was also developed and tested. It is derived from the ratios of bandwidth measured on the time-space diagram and is therefore termed the "band ratio." The advantage of the band ratio is that, unlike the platoon ratio, it may be computed without field studies. From the studies reported in this paper, it appears that the band ratio may be used as a cost-effective substitute for the platoon ratio for most purposes.
Samenvatting