Are Some Freeway Lanes Under-utilized?

Geometric design of a roadway segment affects its throughput and capacity. One of the key factors impacting the basic freeway segment operations in urban areas is the density of ramps. Entry and egress flows associated with ramps create turbulence in the traffic stream and affect the distribution of vehicles across lanes on main freeway segments. The impact of ramps on basic freeway segments is accounted for within the Highway Capacity Manual (HCM) method for estimating free-flow speed (FFS), which is utilized in capacity and level of service estimation. However, the HCM method does not distinguish minor ramps (e.g., arterial-to-freeway) from major ramps (e.g., freeway-to-freeway) and their relative locations (i.e., whether in the upstream or downstream) and does not provide any methods to account for their impacts on the throughput when FFS is known or measured. To better capture the impacts of ramps, lane utilization variation (measured by lane utilization factor) across urban freeway lanes is analyzed and modeled in this paper. This paper explores lane utilization behavior on urban basic freeway segments in high flow situations. Traffic volume and roadway geometric data from 11 basic freeway segment sites in Hampton Roads, Virginia are used for the analysis. Descriptive analysis of the data is provided, followed by rigorous statistical models (ordinary least squares and truncated regression) that are estimated to identify important factors associated with lane utilization variation. Lane utilization behavior is found to be affected differently by ramp types and their relative locations from the freeway segment being analyzed. More specifically, freeway-freeway ramps have more impact than arterial-freeway ramps, and downstream ramps have more impact than their counterparts.