Detailed Observations of Saturation Headways and Start-Up Lost Times

Abstract

The analyses conducted in this research were based on three methodologies for the field measurement of saturation headways. The first method (M1), the one on which most past studies were based, measured the characteristics of Vehicles 4 to 12 in a standing queue. M2, the method found in the Highway Capacity Manual (HCM), counted all vehicles in a standing queue, regardless of queue length. M3 included arrivals that joined the standing queue as long as vehicles were up to 140 ft from the stop line. This study focused on one approach of a high-design intersection with heavy, random arrivals. The large number of observations and the practically ideal traffic conditions enabled the acquisition of several statistically significant results on saturation flow ( s), start-up lost time (SULT), and start-up response time (SRT): ( a) when long queues are present, the typical field measurement of s based on the first 12 vehicles is an overestimate of s for through vehicles and an underestimate of s for protected left-turning vehicles; ( b) the type of movement had a more dominant role in determining s than the level of saturation (or queue length); ( c) SRT displayed a bigger variation than headways— the left-turning movement had a significantly shorter SRT than the through movement did; and ( d) much higher SULTs were estimated in this study compared with those in the HCM.