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HPMS Field Manual

Appendix N: Procedures for Estimating Highway Capacity

Freeway Capacity

Application

All highways (rural and urban) that are "freeway by design" use the following procedures. These are facilities with:

• Four or more through lanes with two-way flow (Data Item 34 >= 4 and Data Item 27 = 2) OR two or more through lanes and one-way flow (Data Item 34 >= 2 and Data Item 27 = 1)
  
  
• Divided Highways - median width >= 4 feet (Data Item 57) or with a "positive" or "curbed" barrier (Data Item 56 = 1 or 2)
  
  
• Access-Controlled Highways (Data Item 55 = 1)

Procedure

Step 1: Calculate Free Flow Speed (FFS)

The first step in the procedure is to estimate free flow speed (FFS) of the facility. HCM Equation 23-1 is applied directly:

Where:

Base Free Flow Speed

BFFS is set at 70 mph for urban facilities and 75 mph for rural facilities.

Adjustment Factor for Lane Width (f_LW)

The values from HCM Exhibit 23-4 are used and are directly based on the values of Data Item 54:

Adjustment Factor for Right Shoulder Lateral Clearance (f_LC)

The values from HCM Exhibit 23-5 (shown as Table 1 here) are used and based directly on the values of Data Item 59. The number of lanes in one direction are computed by halving Data Item 34 for two-way facilities or by using Data Item 34 directly for one-way facilities:

Table 1. Influence of Right Shoulder Widths on FFS

Adjustment Factor for Number of Lanes (f_N)

The values from HCM Exhibit 23-6 are used and based on the number of lanes in one direction. For two-way operation, the number of lanes in one direction is Data Item 34 divided by 2; for one-way facilities the value of Data Item 34 is used directly. The adjustment is made for urban freeways only; for rural facilities f_N is set to 0:

Adjustment Factor for Interchange Density (f_ID)

The number of interchanges is no longer available in HPMS. Therefore, an analysis of 1998 HPMS data was done to determine average interchange densities as a function of functional class and area size (Data Item 13, Rural/Urban Designation). For rural sections, interchange density is assumed not to influence free flow speed. The factor is based on the average interchange densities, as found in the 1998 HPMS data, and linear interpolation of the information in HCM Exhibit 23-7.

Table 2. Influence of Interchange Density on FFS

Step 2: Calculate Base Capacity (BaseCap)

The Base Capacity (passenger cars per hour per lane; pcphpl) of a freeway facility is based on information found in HCM Exhibit 23-3. The following equations were developed based on this information:

Step 3: Determine Peak Capacity (PeakCap)

The HCM 2000 procedure does not make adjustments to the Base Capacity in order to calculate level of service and performance measures. Instead, adjustments are made to the hourly demand volume. However, for HPMS, the capacity of the segment, in terms of total vehicles per hour (vph), must be computed for a variety of analytic purposes. Therefore, the same factors used in the HCM 2000 to adjust volume are used to adjust base capacity instead. Essentially, these adjustments convert the units from passenger cars to vehicles and lower capacity to account for the effect of heavy vehicles. The procedure is based on HCM Equation 23-2:

Where:

Peak Hour Factor (PHF)

The Peak Hour Factor is used to account for variations in flow within the peak hour. The HCM 2000 recommends defaults of 0.92 for urban facilities and 0.88 for rural facilities (Chapter 13). It also states that congested facilities have larger values (0.95 is "typical") than uncongested (unsaturated) ones. Clearly, these factors can have a large impact on capacity. However, determining if an HPMS section is congested is in fact a function of first determining its capacity. Therefore, an iterative process is used:

• Set PHF in Equation 3 equal to 1.0; compute peak capacity
  
  
• Determine an initial volume-to-capacity ratio (V/C), where:
  
  
  • V = AADT * K-Factor * D-Factor (Data Items 33, 85, and 86, respectively, where the K- and D-factors are expressed as decimals)
  • C = Peak Capacity
    
    
• Assign a final PHF as follows:

Table 3. PHF Assignment

Where:

Adjustment Factor for Heavy Vehicles (f_HV)

The adjustment factor for heavy vehicles is based on calculating passenger-car equivalents for trucks and buses. (Recreational vehicles are ignored.) HCM Equation 23-3 and Exhibit 23-8 are used:

Where:

Adjustment Factor for Driver Population (f_p)

For Urban Freeways, the driver population factor is set to 1.0 to indicate that drivers are familiar with roadway and traffic conditions (by virtue of the fact that most of the traffic is composed of commuters). On Rural Freeways, the factor is set to 0.975.