Investigation of Freeway Capacity: a) Effective Capacity of Auxiliary Lanes and b) Segment Capacity as a function of Number of Lanes and Merge/Diverge Activity

Contracting Agency: Florida Department of Transportation (FDOT)

PI: Scott Washburn, Ph.D.

Co-PI: Yafeng Yin, Ph.D.

This project will investigate two freeway capacity issues: 1) the effective capacity of auxiliary lanes, and 2) total segment capacities as a function of the number of lanes.

Effective Capacity of Auxiliary Lanes

Auxiliary lanes[1] are generally used to reduce the traffic turbulence created by merging and diverging movements.  Given that these lanes are used primarily by vehicles either entering or exiting the freeway, it is uncommon for them to realize capacity values similar to those of a regular lane on a basic freeway segment.  However, if the distance between the connecting on-ramp and off-ramp becomes great enough, it is plausible that the auxiliary lane will be used by some amount of through traffic.

The HCM does not offer explicit guidance on the capacity of auxiliary lanes.  The typical interpretation of this issue by analysts is as follows:

• Less than 2500 ft in length, it is analyzed as a weaving section
• Greater than 3000 ft in length, it is considered to have the same capacity as its adjacent regular freeway lanes

These distance thresholds are based on a general interpretation of the distance guidelines given in the HCM related to the analysis of weaving sections and merge/diverge areas.  More specifically, the HCM currently recommends that freeway segments with an auxiliary lane of 2500 ft or less be analyzed with the freeway weaving procedure.  For longer sections, the ramp junctions analysis procedure should be applied to both the on-ramp and off-ramp areas.  The HCM ramp junctions analysis procedure assumes that the influence area of a ramp extends 1500 ft downstream/upstream of an on-ramp/off-ramp.  Thus the selection of the 3000 ft threshold (the combined value of adjacent on-ramp and off-ramp influence areas) for the full-capacity value.  For distance values between 2500 and 3000 ft, analysts assume a wide range of capacity values due to the lack of any guidance whatsoever on this specific distance range.

The FDOT has developed its own guidelines for the capacity of an auxiliary lane, which are currently implemented in its FREEPLAN software program, as follows:

However, the FDOT has not previously conducted a study to validate their selection of these distance-capacity values.

Segment Capacity as a Function of Number of Lanes and Merge/Diverge Activity

The HCM currently relates capacity to free-flow speed (FFS), and the FFS is a function of the number of lanes on the freeway segment.  Exhibit 23-6 specifies that reductions in the free-flow speed (and consequently reductions in capacity) decrease with an increasing number of lanes for the segment (with no further deductions made when the number of lanes, in one direction, totals five or more).  Thus, the implication is that the per lane capacity of a freeway segment increases with a higher number of lanes.

Limited sampling of Florida freeway data (by the Systems Planning Office) has not borne out this tenet of the HCM.  Thus, the FDOT Systems Planning Office is concerned that this aspect of the basic freeways segments analysis procedure of the HCM does not accurately reflect Florida traffic conditions.

Furthermore, Chapter 25 of the HCM indicates that a capacity reduction of 100 pc/h/ln occurs in the two outside (right-side) lanes in a ramp merge area, and a capacity reduction of 200 pc/h/ln occurs in the two outside lanes in a ramp diverge area.  Previously, the HCM did not explicitly indicate whether the total freeway segment capacity would be reduced due to reductions in capacity of the outer lanes in merge/diverge areas.  The FDOT’s interpretation was that capacity reductions in the outer lanes would result in a lower total segment capacity.  For example, for a 4-lane freeway segment (in one direction), with an on-ramp, the total capacity for the segment would be (2300+2300+2400+2400 = 9400), assuming no other capacity reductions.  This interpretation was also consistent with a limited sampling of data from Florida freeways.

Recently, however, the Highway Capacity and Quality of Service (HCQS) committee discussed this particular issue at the request of the FDOT and came to a decision on its interpretation.  The committee’s interpretation was that any reduction in capacity in the outer lanes due to merging or diverging would be compensated for by an increase in capacity in the inner lanes; thus, the total freeway segment capacity would not be affected.  The committee developed language to this effect for Chapter 25 of the HCM that is now included the in errata document.

Again, a limited sampling of data from Florida freeways (by the Systems Planning Office) was not consistent with this recently added guidance to the HCM.

As Florida continues to grow at a rapid pace, and metropolitan areas are undergoing ever-increasing delays on the roadway system, it is essential that the accurate values of capacity be utilized in the FDOT freeway analysis procedure (implemented in FREEPLAN).

Objectives and Supporting Tasks

The objectives of this project are to: 1) develop a research approach that can be used to develop a quantitative relationship for auxiliary lane capacity and the relevant explanatory factors, and 2) determine whether the total number of lanes affects the capacity of a freeway segment and whether merging/diverging traffic affects the total capacity of a freeway segment, for a given number of lanes.  The specific tasks to be conducted in support of accomplishing these objectives are described as follows.

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[1] Auxiliary lanes, as defined in this study, consist of lanes connecting on-ramps to off-ramps.  Furthermore, auxiliary lanes that are physically separated from the adjacent freeway lanes, such as collector-distributor lanes are also not considered.