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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:

HCM Equation

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 = 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
Data Item 88 Data Item 89 No. Peak Lanes Lane Groups No. Lanes in Group
1 1 N/A Exclusive LT 2
Exclusive Through N
Exclusive RT 2
2, 3 Exclusive LT 2
Exclusive Through N
Exclusive RT 1
4 1, 2 Exclusive LT 2
Shared Through/RT N
3+ Exclusive LT 2
Exclusive Through N- 1
Shared Through/RT 1
5 N/A Exclusive LT 2
Exclusive Through N
2,3 1 N/A Exclusive LT 1
Exclusive Through N
Exclusive RT 2
2, 3 Exclusive LT 1
Exclusive Through N
Exclusive RT 1
4 1, 2 Exclusive LT 1
Shared Through/RT N
3+ Exclusive LT 1
Exclusive Through N- 1
Shared Through/RT 1
5 N/A Exclusive LT 1
Exclusive Through N
4 1 1, 2 Shared LT/Through Data Item 87
Exclusive RT 2
3+ Shared LT/Through 1
Exclusive Through N- 1
Exclusive RT 1
2, 3 1, 2 Shared LT/Through N
Exclusive RT 1
3+ Shared LT/Through 1
Exclusive Through N- 1
Exclusive RT 1
4 1, 2 Shared LT/Through/RT N
3+ Shared LT/Through 1
Exclusive Through N- 2
Shared RT/Through 1
5 1, 2 Shared LT/Through N
3+ Shared LT/Through 1
Exclusive Through N- 1
5 1 N/A Exclusive Through N
Exclusive RT 2
2, 3 N/A Exclusive Through N
Exclusive RT 1
4 1, 2 Shared RT/Through N
3+ Shared RT/Through 1
Exclusive Through N- 1
5 N/A Exclusive Through N

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=so N fw fHV fg fp fbb fa fLU fLT fRT fLpb fRpb PHF

where:

S = saturation flow rate for subject lane group, expressed as a total for all lanes in lane group (vph);
So = base saturation flow rate per lane (pcphpl);
N = number of lanes in lane group;
fw = adjustment factor for lane width;
fHV = adjustment factor for heavy vehicles in traffic stream;
fg = adjustment factor for approach grade;
fp = adjustment factor for existence of a parking lane and parking activity adjacent to lane group;
fbb = adjustment factor for blocking effect of local buses that stop within intersection area;
fa = adjustment factor for area type;
fLU = adjustment factor for lane utilization;
fLT = adjustment factor for left turns in lane group;
fRT = adjustment factor for right turns in lane group;
fLpb = pedestrian-bicycle adjustment factor for left-turn movements;
fRpb = pedestrian-bicycle adjustment factor for right-turn movements; and
PHF = Peak Hour Factor.

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, 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 factor for lane width

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:

Adjustment factor for Heavy Vehicles

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 Grade, fg

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

Adjustment for Parking, fp

The calculation of the parking adjustment factor is:

Adjustment factor for Parking

where:

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

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), fp is set to 1.0. It is also set to 1.0 if Data Item 61 = 0.

Adjustment for Bus Blockage, fbb

For HPMS, fbb 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, fa

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. A value of 1.0 is used for rural conditions.

Lane Utilization Adjustment, fLU

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, fLT

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 Protected Phasing, Shared LT (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, PLT 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:

fLT = (1400-Vo)/[(1400-Vo)+(235+0.435*Vo)*PLT] for Vo <= 1,220 vph
  = 1/(1+(4.525*PLT)) for Vo >1,220 vph

Where:

Vo = AADT * K * (1 - D); this is the opposing flow in the off peak direction
AADT = annual average daily traffic, (Data Item 33)
K = K-factor, (Data Item 85) expressed as a decimal
D = the directional factor for the peak direction, (Data Item 86) expressed as a decimal
PLT = proportion of left turns; assume proportion of left turns is 10 percent

Adjustment for Right Turns, fRT

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:

fRT = 0.85 (for Data Item 89 = 1, 2, or 3)
  = 1.0 - 0.15PRT (for Data Item 89 = 4)

where:

PRT = proportion of right turns in the lane group
  = 0.10 (HCM, page 10-19)

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, fLpb

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, fLpb is set to 1.0.

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

Based on the same conditions as for pedestrian-bicycle blockage on left turns, fRpb 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 (gi/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 gi/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

gLT /C = 0.35*g/C for principal arterials
  = 0.25*g/C for all other highway types

For g/C > 0.65

gLT /C = 0.5*(1 - g/C)(all highways)

Where:

g/C = green ratio for through movement (Data Item 91).

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).



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Updated: 04/04/2011
 

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