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HPMS Field Manual

Appendix N: Procedures for Estimating Highway Capacity

Signalized Intersection Capacity

Application

Sections that meet the criteria in the hierarchy for signalized intersection density (> .5 /mile) are analyzed using this procedure. For both rural and urban section where Left Turning Lanes (Data Item 88) and Right Turning Lanes (Data Item 89) are missing, a simplified procedure is used (see page N-23). Data Items 88 and 89 are used to identify lane groups, a critical step in the signalized intersection capacity procedure.

Typical Percent Green Time (Data Item 91) and Peak Parking (Data Item 61) are considered optional for the coding of rural sections. However, default values for these data are provided so that if Data Items 88 and 89 are present, the urban procedure may be used.

Urban Procedure

Signalized Intersection Approach Capacity

For HPMS, the capacity of the entire approach is required, including all movements, primarily for consistency with the speed/delay procedures. Intersection approach capacity is based on HCM Equation 16-6:

where:

This approach is based directly on the H

CA	=	intersection approach capacity
si	=	saturation flow rate for lane group i
	=	effective green ratio for lane group i

CM 2000 principles. When operational analysis is conducted using capacity computed in this way, the HCM 2000 method takes into account the volumes assigned to each lane group. In HPMS, turning movement volumes are not available; only total intersection approach volume is computed from AADT. Therefore, the volume-to-service flow ratio for HPMS is computed using the total intersection approach capacity and volume, rather than considering the operational characteristics of each lane group individually. The HPMS method therefore assumes that traffic is distributed roughly proportionate to the available lane group capacity (i.e., lane groups are neither under-utilized nor over-utilized). While it is possible that this may result in an overestimation of capacity in some instances, there is currently no basis upon which a realistic adjustment can be made. Further, because lane groups are based on the existence of exclusive turning lanes, there is no reason to suspect some lane groups are underutilized at the expense of others; turning lanes are implemented to handle high turning volumes.

Determining Lane Groups

The HCM 2000 methodology for signalized intersections is based on determining capacity for individual lane groups. Lane groups take into account intersection geometry and turning movements. Separate lane groups should be identified for:

• Continuous LT lane or 1 + exclusive LT lanes - LT only lane group
  
  
• Continuous RT lane or 1 + exclusive RT lanes - RT only lane group

In addition to possible exclusive turning lane groups, it is always assumed that through movements occur at the intersection. This assumption does not account for T-intersections, but no intersection geometry data exist in HPMS that would allow this determination. The maximum number of lane groups for HPMS purposes is three. Table 9 is used in developing lane groups, based on the coding of left and right turning lanes.

Table 9. Determining Lane Groups in HPMS

Where:     N = Data Item 87

Determining Saturation Flow Rate

A slightly modified version of HCM Equation 16-4 is used to determine saturation flow rate:

s=s_o N f_w f_HV f_g f_p f_bb f_a f_LU f_LT f_RT f_Lpb f_Rpb PHF

where:

Note that the Peak Hour Factor is included in the calculation of saturation flow rate. The HCM 2000 adjusts volume with this factor, but for HPMS purposes, it is used to adjust saturation flow rate.

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:

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

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 Grade, f_g

For HPMS purposes, f_g is set to 1.0 because of the lack of grade information on urban minor arterials and collectors.

Adjustment for Parking, f_p

The calculation of the parking adjustment factor is:

where:

The parking factor is applied only to lane groups that are immediately adjacent to parking spaces. For two-way streets or one-way streets with parking on one side, this is assumed to occur in the right-most lane group; this will be either an exclusive right-turn lane or a lane with shared movements. For one-way streets with parking on both sides, the parking factor adjustment is made for the left-most and the right-most lane groups.

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 Bus Blockage, f_bb

For HPMS, f_bb is set to 1.0. No data exist in HPMS as to the occurrence of bus routes. Further, the default values in the HCM 2000 for bus maneuvers lead to adjustment factors close to 1.0.

Type of Area Adjustment, f_a

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. A value of 1.0 is used for rural conditions.

Lane Utilization Adjustment, f_LU

The HCM 2000 states that: "As demand approaches capacity, the analyst may use lane utilization factors [close] to 1.0, which would indicate a more uniform use of the available lanes and less opportunity for drivers to freely select their lane" (HCM page 10-26). Because the purpose of this factor in HPMS is to estimate capacity rather than perform an analysis of an individual intersection, a lane utilization adjustment factor of 1.0 is used. This will avoid underestimating capacity in investment analysis where demand volumes are forecast for a long time horizon (usually 20 years).

Adjustment for Left Turns, f_LT

Left turns are a major determinant of intersection approach capacity, yet only limited data are available in HPMS for conducting capacity analyses. The HCM 2000 identifies six cases, as shown in Table 10. The adjustment factor for left turns is applied only if left turns are made from the lane group; this determination is made by checking the coding of Data Item 88, also as shown in Table 10.

Table 10. HCM 2000 Left Turn Adjustment Cases as Applied to HPMS

LT Adjustment Case	Data Item 88	Functional Class	fLT
1. Protected Phasing, Exclusive LT	1, 2, 3	All Arterials; Rural Major Collectors	fLT=0.95
2. Permitted Phasing, Exclusive LT	1, 2, 3	Rural Minor Collectors; Urban Collectors	(see below)
3. Protected/Permitted Phasing, Exclusive LT	N/A	N/A	N/A
4. Protected Phasing, Shared LT	4	Principal Arterials	(PLT= 0.10)
5. Permitted Phasing, Shared LT	4	Minor Arterials; All Collectors	(see below)
6. Protected/Permitted Phasing, Shared LT	N/A	N/A	N/A

A major assumption here is that the phasing is determined by functional class; higher order facilities are assumed to have protected phasing.

Note that Protected/Permitted Phasing is assumed to not exist because of lack of information on signal phasing in HPMS. For Case 4, P_LT is the proportion of left turns in the lane group. As recommended in the HCM 2000, a default value of 10 percent is used for HPMS (HCM page 10-19).

If left turns are not made from the lane group, the left turn adjustment factor is set to 1.0.

Special Procedure for Permissive Left Turns

The permissive left turn procedure in the HCM 2000 is highly complex and dependent on many variables not present in the HPMS data (Appendix C, HCM Chapter 16). The new procedure is substantially more complex than those in previous versions of the HCM. For HPMS purposes, the procedure from the 1994 edition of the HCM is used because it is more compatible with the available data in HPMS:

Where:

Adjustment for Right Turns, f_RT

The adjustment factor for right turns is applied only if right turns are made from the lane group. The right turn adjustment factor is based on the coding of HPMS Data Item 89 as follows:

where:

If right turns are not made from the lane group, the right turn adjustment factor is set to 1.0.

Adjustment for Pedestrian-Bicycle Blockage on Left Turns, f_Lpb

As with permissive left turns, calculation of the adjustment factor for pedestrians and bicycles is complex and requires extensive inputs, most of which are unavailable in HPMS. Further, pedestrian/bicycle blockage is likely to have a major impact only in densely developed areas and these conditions can no longer be determined from HPMS data. Therefore, for the purpose of HPMS, f_Lpb is set to 1.0.

Adjustment for Pedestrian-Bicycle Blockage on Right-Turns, f_Rpb

Based on the same conditions as for pedestrian-bicycle blockage on left turns, f_Rpb is set to 1.0.

Peak Hour Factor (PHF)

As discussed on HCM page 10-8, a default value of 0.92 is used for the PHF for urban sections. For rural sections, the PHF is set to 0.88.

Effective Green Ratios (g_i/C) for Lane Groups

The following values are used for the various types of lane groups. They are based on the coded value for HPMS Data Item 91 (Typical Peak Percent Green Time), which applies to the through movement only. For all lane groups except for exclusive left turn lane groups, the value for Data Item 91 is used for g_i/C. (This assumes that an exclusive signal phase for right turns coincides with the through movement phase.) For Exclusive Left Turn Lane Groups, the green ratio is a function of the green ratio for the through movement (Data Item 91). A distinction is made for low and high values for through green ratio:

For g/C <= 0.65

For g/C > 0.65

Where:

If Data Item 91 is missing, g/C default values of 0.45 for arterials and 0.40 for collectors are used (NCHRP 387, Table 9-3).