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HPMS Field Manual

Appendix N: Procedures for Estimating Highway Capacity

Rural Two-Lane Capacity

Basic Concepts and Approach

In the HCM 2000, capacity is taken as a value that "... is nearly independent of the directional distribution of traffic on a facility, except that for extended lengths of a two-lane highway, the capacity will not exceed 3,200 pc/h for both directions of travel combined." The purpose of the HCM 2000 procedure is to develop two primary performance measures: average travel speed (ATS) and percent time-spent-following. In computing the ATS, v/c ratio, or other performance measures, adjustments are made to volume while the capacity remains fixed at 3,200 passenger cars per hour (pch).

For HPMS, the procedure for estimating ATS is the better of the two for estimating capacity. Equation 20-5 in the HCM 2000 relates ATS to FFS, adjusted flow rate, and the no passing zone factor:

Where:

FFS is developed from a base (ideal) free flow speed and a flow speed adjusted for lane width, shoulder width, and access points. V_p is the two-way hourly volume adjusted for peak 15-minute flows, grades, and heavy vehicles. These concepts are different from previous versions of the HCM where adjustments were made to capacity. However, for use in HPMS and its applications, estimates of capacity are needed. Therefore, the procedure used here is based on adjusting the base capacity for the factors related to V_P and f_np. The adjustments for FFS are not applied to capacity. This avoids double counting when speed and delay methodologies other than those in the HCM 2000 are applied. (These methods usually relate performance measures directly to volume and capacity.)

Application

All rural sections that have partial or no access control and two through lanes of travel and two-way operation are covered by this method.

Procedure

For HPMS purposes estimates of capacity are still needed. Therefore, instead of adjusting flow rates, (volumes) capacity will be adjusted by most of the same factors:

Where:

Grade Adjustment Factor (f_G)

HCM Exhibit 20-7 offers values for f_G but only for level and rolling terrain; it is assumed that the steep grades associated with mountainous terrain will be analyzed individually. For HPMS, this is impractical since a section can have multiple grades: determining an average grade may underestimate the impact while selecting the most severe grade may overestimate the impact. As a compromise, f_Gvalues for grades in the 5.5 - 6.5 percent range with a length of 0.5 miles are assumed for mountainous terrain and HCM Exhibit 20-13 is used. This results in the factors presented in Table 6. The two-way flow rates in Table 6 are based on adjusting the HPMS AADT value to peak passenger cars and is computed as:

Where:

Table 6. Grade Adjustment Factors (f_G) for HPMS

Adjustment for Heavy Vehicles (f_HV)

The HCM 2000 presents two ways of computing passenger car equivalents (pce's) for heavy trucks: one for computing ATS and one for computing percent time-spent-following. For capacity purposes, ATS is considered to be more appropriate. (Percent time-spent-following is more descriptive of driver experience than of physical capacity.) As with grades, mountainous sections are excluded from the default values in the HCM 2000. Therefore, for default purposes, pce's for trucks (E_T) is also taken for grades of 5.5 - 6.5 percent and a length of 0.5 miles using HCM Exhibit 20-15. This results in the values in Table 7.

Table 7. Passenger Car Equivalents for Trucks (E_T)

Once E_T is found, the adjustment factor for heavy vehicles is computed as:

Where:

Note that recreational vehicles are included with single unit trucks in HPMS.

Peak Hour Factor (PHF)

The PHF represents the variation in traffic flow within an hour. A default value of 0.88 should be used for rural two-lane highways.

Adjustment for Percentage of No-Passing Zones (V_NP)

This adjustment is used in the HCM 2000 procedure to compute ATS rather than adjusting flow rate. In Equation 1, f_np can be equilibrated to volume simply by dividing by 0.00776. For example, an f_np of 2.0 translates to 258 passenger cars. For HPMS, this adjustment can also be made to capacity since it too is in terms of passenger cars:

Where:

HCM Exhibit 20-11 should be entered using values for Two-Way Flow Rate (Equation 18). HCM Exhibit 20-11 also requires "% No Passing Zones". This computes as:

Table 8. Adjustment (f_np) for Effect of No-Passing Zones on Average
Travel Speed on Two-Way Segments

Adapted from HCM Exhibit 20-11.