STUDY OF HEADWAY AND LOST TIME AT SINGLE-POINT URBAN INTERCHANGES

Abstract

The results of a recent study of the headway and lost time at three single-point urban interchanges (SPUIs) are summarized. The data base, containing more than 38,000 headway observations, was collected primarily in the Tampa, Florida, area. The data were used to calculate the minimum discharge headway and start-up lost time for the SPUI's three basic movements: cross road left-turn, off-ramp left-turn, and cross road through. It was found that traditional procedures for estimating the minimum discharge headway may be biased toward values higher than ultimately achieved by the traffic queue. Moreover, the degree of bias varied widely among the movements and sites studied because of unequal numbers of observations. As a result, initial attempts at a cause-and-effect analysis were clouded by a high degree of variability in the data. In recognition of the aforementioned bias, alternative statistical analysis techniques and regression models were used to identify significant effects and to calibrate predictive models of minimum discharge headway and start-up lost time. The results indicate that the minimum discharge headway of the SPUI's two left-turn movements are significantly lower than its through movements and lower than values traditionally used for protected left-turn movements under ideal conditions. In fact, the calibrated models predict minimum discharge headways that are generally lower, and start-up lost times that are higher, than those calculated by traditional procedures. Left-turn headway was also found to vary with turn radius.