Evaluating Capacity for Multiple Weaving Areas

The Highway Capacity Manual provides concise, well-defined procedures for estimating capacity and level of service for most types of roadways and intersections. While not perfect, these procedures allow for a quick comparison of roadway geometries when making design decisions. When detailed information is necessary, the analyst can use microscopic simulation; however, there is a stiff cost to doing so. However, performing an operational analysis (e.g., VISSIM) is not always an option in projects due to the higher associated costs. This is mostly the case in smaller projects, in the initial phases of larger projects where there is limited budget available for providing preliminary results, or in projects with low traffic and simple/non-complex geometry. In these cases, having an HCM-based method for evaluating all possible freeway segments becomes rather important. Also, there is pressure to limit right-of-way needed for capacity improvement projects. This constraint could result in the use of multiple weaving areas. The current Highway Capacity Manual (HCM 2016) provides procedures for three types of freeway segments: (1) basic segments away from ramps, (2) merge and diverge segments, and (3) weaving segments, which are formed when a merge is followed closely by a diverge, increasing the amount of lane changing as traffic lines up to for its desired destination. If the influence of a single ramp is felt beyond the next ramp to a second ramp, we have a multiple weave. The configuration is either two entrance ramps followed by an exit ramp or a single entrance ramp followed by two exit ramps. We use the former in this work. The HCM recommends segregating the weaving movements into separate merge, diverge, and simple weaving segments, which ignores the interrelated behavior of the merging and diverging traffic. In this work, we are using microscopic simulation (VISSIM) to find relations between macroscopic variables flow, density, and speed. Capacity is evaluated by gradually raising flows for a range of geometries and flow ratios. Models are developed to express capacity in terms of lane configuration, flow ratios, truck percentage, and overall flow rate. Also, the impact of distances between the ramps and the percent trucks will be measured. We plan to provide a procedure to allow analysts to evaluate capacity in multiple weaves without having to resort to simulation.