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HPMS Field Manual

Appendix N: Procedures for Estimating Highway Capacity

Urban One/Two/Three Lane Highway Capacity

Application

These are used on highway sections in urban areas that do not meet the traffic control device density requirement for either signals or stop signs. They can either have one-way or two-way traffic flow, as follows:

• One-Way, One-Lane Highways: Data Item 34 = 1 and Data Item 27 = 1;
  
  
• Two-Way 2/3- Lane Highways: Data Item 34 = (2 or 3) and Data Item 27 = 2; and
  
  
• Two-Way, One-Lane Highways: Data Item 34 = 2 and Data Item 27 = 1. This is an unusual occurrence but some states code unstriped highways in this manner. For HPMS, it is assumed that these are in fact two-lane highways. In these cases, lane width is determined as the larger of: (Data Item 54 divided by 2) or seven feet.

Many urban sections in HPMS with 1-3 lanes do not have traffic control devices present on the actual section. However, it is possible that these sections are influenced by traffic control devices upstream and downstream of the section. One approach to capacity estimation for these sections is to assume that the capacity is controlled by "off-section" traffic control devices, probably signals. This approach would use the simplified procedure for calculating signalized intersection capacity since the data required for the detailed procedure are lacking. The other approach is to assume that capacity is strictly a function of the section's characteristics, not off-section traffic control devices. Three reasons exist for using the latter assumption. First, it is consistent with the HPMS philosophy of using the coded data to the maximum extent possible. Second, states can provide capacity estimates directly if off-section traffic control devices exert a major influence on the section's capacity. Third, even if off-section traffic control devices exist, the spacing of these may be large enough that they don't affect the section's capacity. For these reasons, it assumed that these sections are not under the influence of traffic control devices.

Procedure

The recommended procedure is based on the adjusted saturation flow rate step from the Signalized Intersection procedure:

Capacity = s₀N f_w f_HV f_p f_aPHF

where:

Base Saturation Flow Rate, S_O

The base saturation flow rate is set at 1,900 pcphpl.

Adjustment Factor for Lane Width, f_W

The lane width adjustment factor is based directly on the HCM 2000 procedure:

where:

Adjustment for Heavy Vehicles, f_HV

The heavy vehicle adjustment factor is based directly on the HCM 2000 procedure, assuming 2 passenger car equivalents for heavy vehicles:

where:

The percent heavy vehicles factor is the sum of peak combination and single unit trucks (Data Items 81 and 83).

Adjustment for Parking, f_p

The calculation of the parking adjustment factor is:

where:

When parking is not allowed or unavailable (Data Item 61= 3), f_p is set to 1.0. It is also set to 1.0 if Data Item 61 = 0.

Adjustment for Area Type

Area type is no longer coded in HPMS. An analysis of 1998 HPMS showed that 9 percent of urban signalized intersections were located in CBDs. The HCM 2000 indicates that f_a should be 0.9 in CBDs, 1.0 elsewhere. Weighting these values with the findings from the 1998 HPMS data provides a value of 0.991 for f_a for urban conditions.

Peak Hour Factor

As discussed on HCM page 10-8, a default value of 0.92 is used for the PHF for urban sections.

HPMS Data Items Used in Capacity Calculations

Volume/Service Flow Ratio (V/SF)

The volume-to-service flow (capacity) ratio is determined for each paved rural sample section and all urban sample sections. It is used as a measurement for congestion. The equations to determine the volume-to-service flow ratio are by type of facility. V/SF is not calculated for a sample section that is entirely on a structure (HPMS, Item 27 = 3,4).

Rural two- or three-lane facility:

V/SF = (AADT (HPMS, Item 33) * K-factor (HPMS, Item 85)) / Peak Capacity
(HPMS, Item 95)

Rural Multilane and All Urban facilities:

V/SF = (AADT (HPMS, Item 33) * K-Factor (HPMS, Item 85) * Directional Factor
(HPMS, Item 86)) / Peak Capacity (HPMS, Item 95)

HPMS Data Items Used in V/SF Ratio