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HPMS Field Manual
Appendix N: Procedures for Estimating Highway Capacity
Urban One/Two/Three Lane Highway Capacity
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:
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.
The recommended procedure is based on the adjusted saturation flow rate step from the Signalized Intersection procedure:
Capacity = s0N fw fHV fp faPHF
Base Saturation Flow Rate, SO
The base saturation flow rate is set at 1,900 pcphpl.
Adjustment Factor for Lane Width, fW
The lane width adjustment factor is based directly on the HCM 2000 procedure:
Adjustment for Heavy Vehicles, fHV
The heavy vehicle adjustment factor is based directly on the HCM 2000 procedure, assuming 2 passenger car equivalents for heavy vehicles:
The percent heavy vehicles factor is the sum of peak combination and single unit trucks (Data Items 81 and 83).
Adjustment for Parking, fp
The calculation of the parking adjustment factor is:
When parking is not allowed or unavailable (Data Item 61= 3), fp 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 fa 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 fa 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
Rural Multilane and All Urban facilities:
V/SF = (AADT (HPMS, Item 33) * K-Factor (HPMS, Item
85) * Directional Factor
HPMS Data Items Used in V/SF Ratio