By Ines Aviles-Spadoni, M.S., STRIDE Research Coordinator

The Highway Capacity Manual (HCM) is a guide for transportation practitioners that includes concepts, guidelines, and computational procedures for calculating the capacity and quality of service for highways, freeways, roads, roundabouts, intersections, and interchanges. It also includes guidance on transit and non-motorized modes such as pedestrians and bicyclists. The first HCM was released in 1950 and is periodically updated with the last major update occurring in January 2022.

The HCM is essentially the go-to manual for transportation engineers and planners. In recent years, however, these practitioners have found that several of the methods cited in Chapters 13 and 27 on weaving segments are unable to capture some sensitivities to important weaving segment parameters operating under certain conditions.

“Weaving segments are developed when an off ramp closely follows an on-ramp on a freeway and are connected via one or more auxiliary lanes,” said Dr. Nagui Rouphail, a professor at North Carolina State University who led Phases 1 and 2 of a STRIDE study on weaving segments’ capacity analysis  in the HCM.

“Turbulence on those sections occur due to the intensive lane changing for both entering and exiting traffic as well as traffic trying to avoid the weaving lanes”. He added that “the more intense lane changing is, the lower the weaving segment capacity and the lower the overall speed of traffic.”

This issue inspired Dr. Rouphail and his research team to undertake two phases of a STRIDE funded project titled Assessing and Addressing Deficiencies in the HCM Weaving Segment Analysis. The work related to the speed and capacity methods contained in the forthcoming STRIDE report are much simpler and accurate compared to the current HCM methods on weaving.

“Gone are separate speed and lane change estimation models for weaving and non-weaving traffic,” Dr. Rouphail said. “The proposed method boils down to a single capacity and speed estimation model.”

The new proposed STRIDE models are sensitive to the weaving configuration details including segment length and lane arrangements and as such are much more applicable for interchange design operational evaluation. They have also been adopted in national research under the NCHRP 7-26 project, which is a collaboration with Kittelson & Associates on combining the data sets and tweaking the models.

“The two teams have agreed to share the field data and recalibrate the STRIDE-NCHRP models with a much larger dataset,” he said. “Once that is accomplished, we will present the final product to the Highway Capacity Committee at TRB and get their approval to include in the next HCM release.

For more information, contact Dr. Nagui Rouphail at rouphail@ncsu.edu or Dr. Ishtiak Ahmed at iahmed2@ncsu.edu