Effect of progression on capacity of interchange ramp weaving

Abstract

Several traffic operational problems are routinely being experienced at one of the most critical elements in the highway system, i.e., grade-separated interchanges. A concern among many practicing engineers is arterial weaving, particularly between the interchange ramp terminal and a closely spaced downstream signalized intersection. High turning volumes and the close spacing between ramp terminals generally associated with interchanges in urbanized areas can create severe operational problems on the cross street. One of the more important factors in determining the level of service of a signalized intersection is the quality of progression along the arterial. The quality of progression is expressed by a progression factor (PF). Simulations were conducted using TRAF-NETSIM to observe the effect of progression on the ramp capacity which showed that the ramp capacity was affected by the quality of progression. The study section consisted of an arterial having an off-ramp on the upstream and a downstream intersection at a distance of 600 feet. There was no signal at the downstream intersection. The ramp traffic, having yield control, made a right turn on to the arterial and then made a left turn at the downstream intersection. All the arterial through traffic was made to go through the downstream intersection without making any turns. Both the default TRAF-NETSIM and Highway Capacity Manual (HCM) recommended decile gap distributions were analyzed. The effect of random flow along the arterial on ramp capacity was studied and a negative exponential regression model was developed to compute ramp capacity for one, two and three lane arterials. Though the trends were found to be similar, due to lower (better) gap acceptance criteria of the HCM recommended decile gap distribution, the ramp capacity while using this distribution was better. The effect of progressed flow on ramp capacity was then studied. It was observed that the variation in ramp capacity for different PF increased with an increase in volume to capacity ratios. Adjustment factors were developed to determine the ramp capacity for different PF given the ramp capacity for random flow.