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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 fw fHV fp faPHF

where:

so = base saturation flow rate per lane (pcphpl);
N = number of peak lanes (Data Item 87);
fw = adjustment factor for lane width;
fHV = adjustment factor for heavy vehicles in traffic stream;
fp = adjustment factor for existence of parking activity;
fa = adjustment factor for area type;
PFH = Peak Hour Factor.

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:

The lane width adjustment factor.

where:

W = Lane Width (Data Item 54); minimum of 8, maximum of 16

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 heavy vehicle adjustment factor

where:

HV = percent heavy vehicles
ET = 2.0 passenger car equivalents

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:

Parking adjustment factor

where:

fp = parking adjustment factor
N= number of lanes in lane group
Nm= number of parking maneuvers per hour
 = 6 for two-way streets for parking on one side
 = 12 for two-way streets with parking on both sides or one-way streets with parking on one side
  = 24 for one-way streets with parking on both sides (based on HCM Exhibit 10 - 20)

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
Item Number Description Item Number Description
17 Functional System 80 Speed Limit
27 Type of Facility 81 Percent Peak Single Unit Trucks
30 Section Length 82 Percent Average Daily Single Unit Trucks
33 AADT 83 Percent Peak Combination Trucks
34 Number of Through Lanes 84 Percent Average Daily Combination Trucks
54 Lane Width 85 K Factor
55 Access Control 86 Directionl Factor
56 Median Type 87 Number of Peak Lanes
57 Median Width 88 Left Turning Lanes
59 Right Shoulder Width 89 Right Turning Lanes
60 Left Shoulder Width 91 Typical Peak Percent Green Time
61 Peak Parking 92 Number At-Grade Intersections - Signals
70 Type of Terrain 93 Number At-Grade Intersections - Stop Signs
78 Percent Passing Sight Distance - Rural 94 Number At-Grade Intersections - Other /No Control
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
Item Number Description
33 AADT
85 K-Factor
86 Directional Factor
95 Peak Capacity


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Updated: 04/04/2011
 

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