APPENDIX N

Procedures for Estimating Highway Capacity

##  {#section .HPMS-Heading-3}

## HPMS Software {#hpms-software .HPMS-Heading-3}

The procedures used in the HPMS software for determining highway
capacity conform to the Highway Capacity Manual 2000 (HCM 2000). The
capacity calculations are based on service flow rates for level of
service E and are for the peak direction. The capacity coded in HPMS is
used for system planning analysis, not project level analysis. The
number of peak lanes (number of through lanes used in the peak period in
the peak direction) coded in HPMS (Item 87) is used in the procedures
for determining capacity. The number of through lanes coded in HPMS
(Item 34) is used in the procedures to determine the number of lanes on
the facility. The equations for determining the volume/service flow
ratio (V/SF) are shown at the end of this Appendix along with tables
that contain the data items used in the capacity calculations and in the
V/SF ratio.

All references to chapters, tables, etc., are to the HCM. All
calculations and values in the Appendix are in English units; i.e.,
miles per hour (mph), feet, miles, etc. The assumptions made by FHWA for
adjustment factors used in the procedures are consistent with the
recommended values in the HCM. The reference to the data item value in
the procedures indicates the way the data item is coded in the HPMS.

Hierarchy Of Application For Capacity Procedures

Figure 1 presents the decision rules for applying the methodologies of
measuring capacity for highway sections. The logic is based on first
identifying if the section is on a structure or is a rural unpaved road;
and second if it is characterized by "interrupted flow;" that is, a
section where traffic is influenced by traffic control devices (signals
and stop signs).

If the traffic control device density is below the thresholds (0.5
signals or stop signs per mile), the facility is assumed to be an
uninterrupted flow facility. For these, the following definitions apply.

Freeways (Urban and Rural)

For HPMS, the design characteristics, not the functional classification,
determine if a section is a freeway ("freeway by design"). The
characteristics of a freeway are:

-   Four or more through lanes with two-way flow (Data Item 34 \>= 4 and
    Data Item 27 = 2) **OR** two or more through lanes and one-way flow
    (Data Item 34 \>= 2 and Data Item 27 = 1)

-   Divided Cross-Section -- for sections with two-way traffic flow,
    median width \>= 4 feet (Data Item 57) or with a "positive" or
    "curbed" barrier (Data Item 56 = 1 or 2) (not applicable for one-way
    sections) and

-   Full Control of Access (Data Item 55 = 1)

Multilane Highways (Urban and Rural)

The main characteristic distinguishing these facilities from freeways is
the lack of access control. They are defined by:

-   Partial or no access control (Data Item 55 = 2 or 3) with a Divided
    Cross-Section (see above under "Freeways") **OR** Full Access
    Control and Undivided Cross-Section; and

-   4 or more through lanes and two-way operation (Data Item 34 \>= 4
    and Data Item 27 = 2) **OR** 2 or more through lanes and one-way
    operation (Data Item 34 = 2 or 3 and Data Item 27 = 1)

Rural Two-Lane Highways

All rural sections that have two through lanes of travel and two-way
operation are covered by this method. Most of these sections will not
have full access control, though this condition does exist in the HPMS
data.

Rural One-Lane, One-Way Highways

This is a rare condition in HPMS, but it does exist.

Rural Three-Lane Highways

These are highways with two through lanes in one direction and one
through lane in the opposite direction.

Urban One/Two/Three-Lane Highways

These are 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.

![](media/image1.wmf)Figure 1. Capacity Hierarchy

Freeway Capacity

## Application {#application .HPMS-Heading-3}

All highways (rural and urban) that are "freeway by design" use the
following procedures. These are facilities with:

-   Four or more through lanes with two-way flow (Data Item 34 \>= 4 and
    Data Item 27 = 2) **OR** two or more through lanes and one-way flow
    (Data Item 34 \>= 2 and Data Item 27 = 1)

-   Divided Highways -- median width \>= 4 feet (Data Item 57) or with a
    "positive" or "curbed" barrier (Data Item 56 = 1 or 2)

-   Access-Controlled Highways (Data Item 55 = 1)

## Procedure {#procedure .HPMS-Heading-3}

Step 1: Calculate Free Flow Speed (FFS)

The first step in the procedure is to estimate free flow speed (FFS) of
the facility. *HCM* Equation 23-1 is applied directly:

*FFS* = *BFFS* -- *f~LW~* - *f~LC~* -- *f~N~* - *f~ID~* (1)

Where:

*BFFS* = base free flow speed

*f~LW\ ~* = adjustment factor for lane width

*f~LC\ ~* = adjustment factor for right shoulder lateral clearance

*f~N~* = adjustment factor for number of lanes

*f~ID\ ~* = adjustment factor for interchange density

**Base Free Flow Speed**

*BFFS* is set at 70 mph for urban facilities and 75 mph for rural
facilities.

**Adjustment Factor for Lane Width (*f~LW~*)**

The values from *HCM* Exhibit 23-4 are used and are directly based on
the values of Data Item 54:

[Lane Width]{.underline} [Reduction in FFS (mph; *f~LW~*)]{.underline}

12 ft. 0.0

11 ft. 1.9

\<=10 ft. 6.6

**Adjustment Factor for Right Shoulder Lateral Clearance
(*f~LC~****[)]{.underline}*

The values from *HCM* Exhibit 23-5 (shown as Table 1 here) are used and
based directly on the values of Data Item 59. The number of lanes in one
direction are computed by halving Data Item 34 for two-way facilities or
by using Data Item 34 directly for one-way facilities:

Table 1. Influence of Right Shoulder Widths on FFS

  -------------------- -------------------------- --------------------- --------------------- ------------------------
  **Right\             **Reduction in FFS (mph;                                               
  Shoulder\            *f~LC~) ***                                                            
  Width**                                                                                     

                       **[Lanes in One                                                        
                       Direction]{.smallcaps}**                                               

                       **[2]{.smallcaps}**        **[3]{.smallcaps}**   **[4]{.smallcaps}**   **[\>=5]{.smallcaps}**

  \>=6                 0.0                        0.0                   0.0                   0.0

  5                    0.6                        0.4                   0.2                   0.1

  4                    1.2                        0.8                   0.4                   0.2

  3                    1.8                        1.2                   0.6                   0.3

  2                    2.4                        1.6                   0.8                   0.4

  1                    3.0                        2.0                   1.0                   0.5

  0                    3.6                        2.4                   1.2                   0.6
  -------------------- -------------------------- --------------------- --------------------- ------------------------

Adjustment Factor for Number of Lanes (*f~N~*)

The values from *HCM* Exhibit 23-6 are used and based on the number of
lanes in one direction. For two-way operation, the number of lanes in
one direction is Data Item 34 divided by 2; for one-way facilities the
value of Data Item 34 is used directly. The adjustment is made for urban
freeways only; for rural facilities *f~N~* is set to 0:

No. Lanes (One Direction;

> [Urban Only)]{.underline} [Reduction in FFS (mph; *f~N~)*]{.underline}

\>= 5 0.0

4.  1.5

> 3 3.0

2.  4.5

Adjustment Factor for Interchange Density (*f~ID~*)

The number of interchanges is no longer available in HPMS. Therefore, an
analysis of 1998 HPMS data was done to determine average interchange
densities as a function of functional class and area size (Data Item 13,
Rural/Urban Designation). For rural sections, interchange density is
assumed not to influence free flow speed. The factor is based on the
average interchange densities, as found in the 1998 HPMS data, and
linear interpolation of the information in *HCM* Exhibit 23-7.

Table 2. Influence of Interchange Density on FFS

  ----------------- ----------------- ----------------- -----------------
  **Functional      **Area Size**     **Interchange     **Interchange
  Class**                             Density**         Adj. Factor,
                                                        (*f~ID~*)**

  Urban Interstates Small Urban       0.70              1.0

                    Small Urbanized   0.76              1.3

                    Large Urbanized   0.83              1.7

  Other Urban       Small Urban       0.83              1.7
  Highways                                              
  Qualifying as                                         
  Freeways                                              

                    Small Urbanized   0.88              1.9

                    Large Urbanized   0.91              2.1
  ----------------- ----------------- ----------------- -----------------

Step 2: Calculate Base Capacity (*BaseCap*)

The Base Capacity (passenger cars per hour per lane; pcphpl) of a
freeway facility is based on information found in *HCM* Exhibit 23-3*.*
The following equations were developed based on this information:

*BaseCap =* 1,700 + 10*FFS*; for *FFS* \<= 70 (2)

*BaseCap =* 2,400; for *FFS* \> 70

Step 3: Determine Peak Capacity (*PeakCap*)

The *HCM 2000* procedure does not make adjustments to the Base Capacity
in order to calculate level of service and performance measures.
Instead, adjustments are made to the hourly demand volume. However, for
HPMS, the capacity of the segment, in terms of total vehicles per hour
(vph), must be computed for a variety of analytic purposes. Therefore,
the same factors used in the *HCM 2000* to adjust volume are used to
adjust base capacity instead. Essentially, these adjustments convert the
units from passenger cars to vehicles and lower capacity to account for
the effect of heavy vehicles. The procedure is based on *HCM* Equation
23-2:

*PeakCap* = *BaseCap* \* *PHF* \* *N* \* *f~HV~* \* *f~p~* (3)

Where:

*PeakCap* = HPMS Peak Capacity (Data Item 95), vehicles per hour (all
lanes, one direction)

*PHF* = Peak Hour Factor

*N* = Number of lanes in one direction

= Number of Peak Lanes (Data Item 87)

*f~HV~* = Adjustment factor for heavy vehicles

*f~p~* = Adjustment factor for driver population

Peak Hour Factor (PHF)

The Peak Hour Factor is used to account for variations in flow within
the peak hour. The *HCM 2000* recommends defaults of 0.92 for urban
facilities and 0.88 for rural facilities (Chapter 13). It also states
that congested facilities have larger values (0.95 is "typical") than
uncongested (unsaturated) ones. Clearly, these factors can have a large
impact on capacity. However, determining if an HPMS section is congested
is in fact a function of first determining its capacity. Therefore, an
iterative process is used:

-   Set PHF in Equation 3 equal to 1.0; compute peak capacity

-   Determine an initial volume-to-capacity ratio (V/C), where:

```{=html}
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```
-   V = AADT \* K-Factor \* D-Factor (Data Items 33, 85, and 86,
    respectively, where the K- and D-factors are expressed as decimals)

-   C = Peak Capacity

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```
-   Assign a final PHF as follows:

Table 3. PHF Assignment

  -------------------- --------------------------- -----------------------
  **Area Type**        **V/C Ratio**               **PHF**

  Rural                \< 0.7744                   0.88

                       0.7744\<=V/C\<=0.9025       Equation (4)

                       \> 0.9025                   0.95

  Urban                \< 0.8100                   0.90

                       0.8100\<=V/C\<=0.9025       Equation (4)

                       \> 0.9025                   0.95
  -------------------- --------------------------- -----------------------

Where: *PHF* = (0.9025 \* V/C)^0.5^/0.95 for special cases above (4)

Adjustment Factor for Heavy Vehicles (*f~HV~*)

The adjustment factor for heavy vehicles is based on calculating
passenger-car equivalents for trucks and buses. (Recreational vehicles
are ignored.) *HCM* Equation 23-3 and Exhibit 23-8 are used:

*f~HV~* = [1]{.underline}

1 + *P~T~*(*E~T~* -- 1) (5)

Where:

*P~T~* = Proportion of trucks and buses in the traffic stream, expressed
as a decimal (e.g., 0.15 for 15%)

= (Percent Peak Combination Trucks, Data Item 83 + Percent Peak Single
Unit Trucks, Data Item 81)

*E~T~* = Passenger-car equivalents

= 1.5 for all urban freeways

= 1.5 for rural freeways in level terrain (Data Item 70 = 1)

= 2.5 for rural freeways in rolling terrain (Data Item 70 = 2)

= 4.5 for rural freeways in mountainous terrain (Data Item 70 = 3)

Adjustment Factor for Driver Population (*f~p~)*

For Urban Freeways, the driver population factor is set to 1.0 to
indicate that drivers are familiar with roadway and traffic conditions
(by virtue of the fact that most of the traffic is composed of
commuters). On Rural Freeways, the factor is set to 0.975.

Multilane Highway Capacity

## Application {#application-1 .HPMS-Heading-3}

The following procedures are applied to rural and urban multilane
highways with the following characteristics:

-   Partial or no access control (Data Item 55 = 2 or 3) with a Divided
    Cross-Section (see above under "Freeways") **OR** Full Access
    Control and Undivided Cross-Section; and

-   4 or more through lanes and two-way operation (Data Item 34 \>= 4
    and Data Item 27 = 2 ) **OR** 2-3 through lanes and one-way
    operation (Data Item 34 = 2 or 3 and Data Item 27 = 1.

## Procedure {#procedure-1 .HPMS-Heading-3}

Step 1: Calculate Free Flow Speed (FFS)

The first step in the procedure is to estimate free flow speed (FFS) of
the facility. *HCM* Equation 21-1 is applied directly:

*FFS* = *BFFS* -- *f~LW~* - *f~LC~* -- *f~M~* -- *f~A~* (6)

Where:

*BFFS* = base free flow speed

*f~LW\ ~* = adjustment factor for lane width

*f~LC~* = adjustment factor for lateral clearance

*f~M\ ~* = adjustment factor for median type

*f~A\ ~* = adjustment factor for access points

Base Free Flow Speed

Base Free Flow Speed is based on the coded speed limit (Data Item 80)
and guidance from the *HCM 2000*. To be consistent with the *HCM 2000*
methodology, the BFFS is not allowed to go below 40 mph or above 70 mph.
This conflicts with guidance in the *HCM 2000* which states that the
methodology is valid for free flow speeds between 45 mph and 60 mph.
However, the *HCM 2000* methodology is geared to estimating performance
characteristics, not capacity; for the purpose of capacity, these
restrictions were relaxed.

*BFFS* = 40 mph, for posted speed limits \< 40 mph (7)

= *SpeedLimit* + 7, for posted speed limits 40-45 mph

= *SpeedLimit* + 5, for posted speed limits \>= 50 mph

Where: *SpeedLimit* is the value for Data Item 80.

Adjustment Factor for Lane Width (*f~LW~*)

The values from *HCM* Exhibit 21-4 are used and directly based on the
values of Data Item 54:

[Lane Width]{.underline} [Reduction in *FFS* (mph; *f~LW~*)]{.underline}

12 ft. 0.0

11 ft. 1.9

\<=10 ft. 6.6

Adjustment Factor for Lateral Clearance (*f~LC~*)

The values from *HCM* Exhibit 21-5 are used and based on computing Total
Lateral Clearance (*TLC*) using right and left shoulder widths. Lateral
Clearance Left (*LC~L~*) is computed for divided highways only (Data
Item 57 \>= 4 or Data Item 56 = 1 or Data Item 56 = 2). In all other
cases, such as if a continuous two-way left turn lane exists (Data Item
88 = 2) or the facility is undivided, then *LC~L~* is set to 6. (The
median type adjustment factor accounts for the fact that the roadway is
undivided.) Facilities with one-way traffic operation are considered
divided highways since there is no opposing flow to interfere with
traffic. For one-way facilities, left shoulder width is not coded.
Therefore, a value of 6 is assumed for one-way facilities.

*TLC* = *LC~R~* + *LC~L~* (8)

Where:

*LC~R~* = Data Item 59, maximum value of 6

*LC~L~* = Data Item 60, maximum value of 6

Once *TLC* is computed, the values in *HCM* Exhibit 21-5 are used.
Linear interpolation is used for intermediate values:

Table 4. Adjustment for Lateral Clearance

+-----------------+-----------------+-----------------+-----------------+
| **4-Lane        |                 | **6+Lane        |                 |
| Two-Way         |                 | Two-Way         |                 |
| Highways and**  |                 | Highways and**  |                 |
|                 |                 |                 |                 |
| **2-Lane        |                 | **3+Lane        |                 |
| One-way         |                 | One-Way         |                 |
| Highways**      |                 | Highways**      |                 |
+-----------------+-----------------+-----------------+-----------------+
| **[Total        | **[Reduction in | **[Total        | **[Reduction in |
| Lateral         | *FFS*           | Lateral         | *FFS*           |
| Clearance       | ]{.smallcaps}** | Clearance       | ]{.smallcaps}** |
| ]{.smallcaps}** |                 | ]{.smallcaps}** |                 |
+-----------------+-----------------+-----------------+-----------------+
| 12              | 0.0             | 12              | 0.0             |
+-----------------+-----------------+-----------------+-----------------+
| 10              | 0.4             | 10              | 0.4             |
+-----------------+-----------------+-----------------+-----------------+
| 8               | 0.9             | 8               | 0.9             |
+-----------------+-----------------+-----------------+-----------------+
| 6               | 1.3             | 6               | 1.3             |
+-----------------+-----------------+-----------------+-----------------+
| 4               | 1.8             | 4               | 1.7             |
+-----------------+-----------------+-----------------+-----------------+
| 2               | 3.6             | 2               | 2.8             |
+-----------------+-----------------+-----------------+-----------------+
| 0               | 5.4             | 0               | 3.9             |
+-----------------+-----------------+-----------------+-----------------+

Adjustment Factor for Median Type (*f~M~)*

The values from *HCM* Exhibit 21-6 are used and based on whether the
facility is divided or undivided. (See definition under Lateral
Clearance Adjustment).

[Highway Type]{.underline} [Reduction in FFS (mph; *f~M~)*]{.underline}

Undivided 1.6

Divided (including TWLTLs[^1]) 0.0

Adjustment Factor for Access Points (*f~A~*)

The number of access points per mile is based directly on the values of
the number of at-grade intersections with no traffic control devices
(Data Item 94) and section length (Data Item 30). In addition, default
values for driveways per mile are also assumed. A linear equation was
fit to the data in *HCM* Exhibit 21-7 and produces:

*f~A~ =* MIN*((0.25\*AccessPointDensity), 10)* (9)

Where:

*AccessPointDensity* = (Data Item 94/Data Item 30) + *DrivewayDensity*

*DrivewayDensity* = 2 for divided highways

= 3 for undivided highways

Step 2: Calculate Base Capacity (BaseCap)

The Base Capacity (passenger cars per hour per lane; pcphpl) of a
multilane facility is based on information found in *HCM* Exhibit
21-3*.* The following equations were developed based on this
information:

*BaseCap =* 1,000 + 20*FFS*; for *FFS* \<= 60 (10)

*BaseCap =* 2,200; for *FFS* \> 60

Step 3: Determine Peak Capacity (PeakCap)

The *HCM 2000* procedure does not make adjustments to the Base Capacity
in order to calculate level of service and performance measures.
Instead, adjustments are made to the hourly demand volume. However, for
HPMS, the capacity of the section, in terms of total vehicles per hour
(vph), must be computed for a variety of analytic purposes. Therefore,
the same factors used in the *HCM 2000* to adjust volume are used to
adjust base capacity. Essentially, these adjustments convert the units
from passenger cars to vehicles and lower capacity to account for the
effect of heavy vehicles. The procedure is based on *HCM* Equation 21-3:

*PeakCap* = *BaseCap* \* *PHF* \* *N* \* *f~HV~* \* *f~p\ ~* (11)

Where:

*PeakCap* = HPMS Peak Capacity (Data Item 95), vehicles per hour (all
lanes, one direction)

*PHF* = Peak Hour Factor

*N* = Number of lanes in one direction

= Number of Peak Lanes (Data Item 87)

*f~HV~* = Adjustment factor for heavy vehicles

*f~p~* = Adjustment factor for driver population

= 1.0 for HPMS

Peak Hour Factor (*PHF*)

The Peak Hour Factor is used to account for variations in flow within
the peak hour. The *HCM 2000* recommends defaults of 0.92 for urban
facilities and 0.88 for rural facilities (Chapter 13). It also states
that congested facilities have larger values (0.95 is "typical") than
uncongested (unsaturated) ones. Clearly, these factors can have a large
impact on capacity. However, determining if an HPMS section is congested
is in fact a function of first determining its capacity. Therefore, an
iterative process is used:

-   Set *PHF* in Equation 6 equal to 1.0; compute peak capacity

-   Determine an initial volume-to-capacity ratio (V/C), where:

```{=html}
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```
-   V = AADT \* K-Factor \* D-Factor (Data Items 33, 85, and 86,
    respectively)

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```
-   C = Peak Capacity

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```
-   Assign a final *PHF* as follows:

Table 5. Final *PHF* Assignment

  -------------------- --------------------------- -----------------------
  **Area Type**        **V/C Ratio**               **PHF**

  Rural                \< 0.7744                   0.88

                       0.7744\<=V/C\<=0.9025       Equation (12)

                       \> 0.9025                   0.95

  Urban                \< 0.8100                   0.90

                       0.8100\<=V/C\<=0.9025       Equation (12)

                       \> 0.9025                   0.95
  -------------------- --------------------------- -----------------------

Where: *PHF* = (0.9025 \* V/C)^0.5^/0.95 for special cases above (12)

Adjustment Factor for Heavy Vehicles (*f~HV~*)

The adjustment factor for heavy vehicles is based on calculating
passenger-car equivalents for trucks and buses. (Recreational vehicles
are ignored.) *HCM* Equation 23-3 and Exhibit 23-8 are used:

*f~HV~* = [1]{.underline}

1 + *P~T~*(*E~T~* -- 1) (13)

Where:

*P~T~* = Proportion of trucks and buses in the traffic stream, expressed
as a decimal (e.g., 0.15 for 15%)

= (Percent Peak Combination Trucks {Data Item 83}) + (Percent Peak
Single Unit Trucks {Data Item 81})

*E~T~* = Passenger-car equivalents

= 1.5 for all urban highways

= 1.5 for rural highways in level terrain (Data Item 70 = 1)

= 2.5 for rural highways in rolling terrain (Data Item 70 = 2)

= 4.5 for rural highways in mountainous terrain (Data Item 70 = 3)

Rural Two-Lane Capacity

## Basic Concepts and Approach {#basic-concepts-and-approach .HPMS-Heading-3}

In the *HCM 2000*, capacity is taken as a value that "... is nearly
independent of the directional distribution of traffic on a facility,
except that for extended lengths of a two-lane highway, the capacity
will not exceed 3,200 pc/h for both directions of travel combined." The
purpose of the *HCM 2000* procedure is to develop two primary
performance measures: average travel speed (ATS) and percent
time-spent-following. In computing the ATS, v/c ratio, or other
performance measures, adjustments are made to volume while the capacity
remains fixed at 3,200 passenger cars per hour (pch).

For HPMS, the procedure for estimating ATS is the better of the two for
estimating capacity. Equation 20-5 in the *HCM 2000* relates ATS to FFS,
adjusted flow rate, and the no passing zone factor:

*ATS* = *FFS -- 0.00776V~P~ -- f~np~* (14)

Where:

*ATS* = Average travel speed

*V~P~* = passenger car equivalent flow rate for peak 15-minutes

*f~np\ ~*= no passing zone adjustment factor from Table 20-11

*FFS* is developed from a base (ideal) free flow speed and a flow speed
adjusted for lane width, shoulder width, and access points. *V~p~* is
the two-way hourly volume adjusted for peak 15-minute flows, grades, and
heavy vehicles. These concepts are different from previous versions of
the *HCM* where adjustments were made to capacity. However, for use in
HPMS and its applications, estimates of capacity are needed. Therefore,
the procedure used here is based on adjusting the base capacity for the
factors related to *V~P~* and *f~np~*. The adjustments for *FFS* are
**not** applied to capacity. This avoids double counting when speed and
delay methodologies other than those in the *HCM 2000* are applied.
(These methods usually relate performance measures directly to volume
and capacity.)

## Application {#application-2 .HPMS-Heading-3}

All rural sections that have partial or no access control and two
through lanes of travel and two-way operation are covered by this
method.

## Procedure  {#procedure-2 .HPMS-Heading-3}

For HPMS purposes estimates of capacity are still needed. Therefore,
instead of adjusting flow rates, (volumes) capacity will be adjusted by
most of the same factors:

*Two-Way* *Capacity = (3,200 pch \* PHF \* f~G~ \* f~HV~) - V~NP~* (15)

Where:

*PHF* = Peak Hour Factor

= 0.88

*f~G~* = Adjustment factor for grades

*f~HV~* = Adjustment factor for heavy vehicles

*V~NP~* = Volume adjustment for no passing zones

Grade Adjustment Factor (*f~G~)*

*HCM* Exhibit 20-7 offers values for *f~G~* but only for level and
rolling terrain; it is assumed that the steep grades associated with
mountainous terrain will be analyzed individually. For HPMS, this is
impractical since a section can have multiple grades: determining an
average grade may underestimate the impact while selecting the most
severe grade may overestimate the impact. As a compromise, *f~G~* values
for grades in the 5.5 -- 6.5 percent range with a length of 0.5 miles
are assumed for mountainous terrain and *HCM* Exhibit 20-13 is used.
This results in the factors presented in Table 6. The two-way flow rates
in Table 6 are based on adjusting the HPMS AADT value to peak passenger
cars and is computed as:

Two-Way Flow Rate = *(AADT \* K)/f~HVD\ ~* (16)

Where:

*AADT* = Data Item 33

*K* = Data Item 85, expressed as a decimal

*f~HVD~* = Daily heavy vehicle adjustment factor

= 1/(1 + 0.5*P~T~)*

*P~T~* = Percentage of daily trucks

= (Data Item 82 + Data Item 84) \* 0.01

Table 6. Grade Adjustment Factors (*f~G~*) for HPMS

  ------------------------- ------------------------- --------------------------- -------------------------------
  **Two-Way Flow Rates**                                                          

  **[(pch)]{.smallcaps}**   **[Level]{.smallcaps}**   **[Rolling]{.smallcaps}**   **[Mountainous]{.smallcaps}**

  0-600                     1.00                      0.71                        0.57

  \>600-1,200               1.00                      0.93                        0.85

  \>1,200                   1.00                      0.99                        0.99
  ------------------------- ------------------------- --------------------------- -------------------------------

[Adjustment for Heavy Vehicles (*f~HV~*)]{.underline}

The *HCM 2000* presents two ways of computing passenger car equivalents
(pce's) for heavy trucks: one for computing ATS and one for computing
percent time-spent-following. For capacity purposes, ATS is considered
to be more appropriate. (Percent time-spent-following is more
descriptive of driver experience than of physical capacity.) As with
grades, mountainous sections are excluded from the default values in the
*HCM 2000*. Therefore, for default purposes, pce's for trucks (E~T~) is
also taken for grades of 5.5 -- 6.5 percent and a length of 0.5 miles
using *HCM* Exhibit 20-15. This results in the values in Table 7.

Table 7. Passenger Car Equivalents for Trucks (*E~T~*)

  ------------------------- ------------------------- --------------------------- -------------------------------
  **Two-Way Flow Rates**    **Type of Terrain**                                   

  **[(pch)]{.smallcaps}**   **[Level]{.smallcaps}**   **[Rolling]{.smallcaps}**   **[Mountainous]{.smallcaps}**

  0-600                     1.7                       2.5                         7.2

  \>600-1,200               1.2                       1.9                         7.2

  \>1,200                   1.1                       1.5                         7.2
  ------------------------- ------------------------- --------------------------- -------------------------------

Once E~T~ is found, the adjustment factor for heavy vehicles is computed
as:

*f~HV~* = [1]{.underline}

1 + *P~T~*(*E~T~* -- 1) (17)

Where:

> *P~T~* = proportion of trucks (combinations plus single units) from
> HPMS, expressed as a decimal

= (Percent Peak Combination Trucks, Data Item 83 + Percent Peak Single
Unit Trucks, Data Item 81)

*E~T\ ~*= Passenger car equivalents from Table 7.

Note that recreational vehicles are included with single unit trucks in
HPMS.

Peak Hour Factor (*PHF*)

The *PHF* represents the variation in traffic flow within an hour. A
default value of 0.88 should be used for rural two-lane highways.

Adjustment for Percentage of No-Passing Zones (*V~NP~*)

This adjustment is used in the *HCM 2000* procedure to compute ATS
rather than adjusting flow rate. In Equation 1, *f~np~* can be
equilibrated to volume simply by dividing by 0.00776. For example, an
*f~np\ ~* of 2.0 translates to 258 passenger cars. For HPMS, this
adjustment can also be made to capacity since it too is in terms of
passenger cars:

*V~NP~ = f~NP~*/0.00776 (18)

Where:

*f~NP~ =* values from *HCM* Exhibit 20-11 (given here as Table 8)

*HCM* Exhibit 20-11 should be entered using values for Two-Way Flow Rate
(Equation 18). *HCM* Exhibit 20-11 also requires "% No Passing Zones".
This computes as:

*% No Passing Zones =* 1 -- (Data Item 78 \* 0.01) (19)

Table 8. Adjustment (*f~np~*) for Effect of No-Passing Zones on Average
Travel Speed on Two-Way Segments

  ------------------------- ------------- -------- -------- -------- -------- -------- -------- -------- -------- -------- ---------
  **[Two-Way                **Reduction                                                                                    
  Demand]{.smallcaps}**     in Average                                                                                     
                            Travel Speed                                                                                   
                            (mi/h)\                                                                                        
                            No-Passing                                                                                     
                            Zones (%)**                                                                                    

  **[Flow Rate,             **0**         **10**   **20**   **30**   **40**   **50**   **60**   **70**   **80**   **90**   **100**
  vp(pc/h)]{.smallcaps}**                                                                                                  

  0-100                     0.0           0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0      0.0

  101-300                   0.0           0.3      0.6      1.0      1.4      1.9      2.4      2.5      2.6      3.1      3.5

  301-500                   0.0           0.9      1.7      2.2      2.7      3.1      3.5      3.7      3.9      4.2      4.5

  501-700                   0.0           0.8      1.6      2.0      2.4      2.7      3.0      3.2      3.4      3.7      3.9

  701-900                   0.0           0.7      1.4      1.7      1.9      2.2      2.4      2.6      2.7      2.9      3.0

  901-1,100                 0.0           0.6      1.1      1.4      1.6      1.8      2.0      2.1      2.2      2.4      2.6

  1,101-1,300               0.0           0.4      0.8      1.0      1.2      1.4      1.6      1.8      1.9      2.0      2.1

  1,301-1,500               0.0           0.3      0.6      0.8      0.9      1.1      1.2      1.3      1.4      1.6      1.7

  1,501-1,700               0.0           0.3      0.6      0.7      0.8      1.0      1.1      1.2      1.3      1.4      1.5

  1,701-1,900               0.0           0.3      0.5      0.6      0.7      0.9      1.0      1.1      1.1      1.2      1.3

  1,901-2,100               0.0           0.3      0.5      0.6      0.6      0.8      0.9      1.0      1.0      1.1      1.1

  2,101-2,300               0.0           0.3      0.5      0.6      0.6      0.8      0.9      0.9      0.9      1.0      1.1

  2,301-2,500               0.0           0.3      0.5      0.6      0.6      0.7      0.8      0.9      0.9      1.0      1.1

  2,501-2,700               0.0           0.3      0.5      0.6      0.6      0.7      0.8      0.9      0.9      1.0      1.0

  2,701-2,900               0.0           0.3      0.5      0.6      0.6      0.7      0.7      0.8      0.8      0.9      0.9

  2,901-3,100               0.0           0.3      0.5      0.6      0.6      0.7      0.7      0.7      0.7      0.8      0.8

  3,101-3,300               0.0           0.3      0.5      0.6      0.6      0.6      0.6      0.6      0.6      0.7      0.7

  \>3,300                   0.0           0.3      0.5      0.5      0.5      0.5      0.5      0.5      0.5      0.5      0.5

                                                                                                                           
  ------------------------- ------------- -------- -------- -------- -------- -------- -------- -------- -------- -------- ---------

Adapted from *HCM* Exhibit 20‑11.

Rural One-Lane Capacity

## Application {#application-3 .HPMS-Heading-3}

These highways, though rare, do exist in HPMS. They include facilities
with both one-way and two-way operation.

## Procedure {#procedure-3 .HPMS-Heading-3}

One-Way Operation

The capacity of these sections should be computed using the procedure
for Rural Two-Lane Highways, with the following changes:

-   Change the base capacity from 3,200 pch to 1,600 pch

-   Compute the one-way flow rate using the same equation as for two-way
    flow rate; enter all lookup tables and equations labeled for two-way
    flow rate with this value.

-   Assume 100% no passing zones.

The resulting capacity is, by definition, a one-way value.

Two-Way Operation

One-lane highways open to both directions of traffic are extremely rare.
For these cases, compute the capacity for one-way operation and divide
by 2.

Rural Three-Lane Highway Capacity

## Application {#application-4 .HPMS-Heading-3}

These highways have two through lanes in one direction and a single lane
in the opposite direction. The through lanes are assumed to cover an
extended length and are not auxiliary or truck climbing lanes. (This is
consistent with the coding of HPMS data).

## Procedure {#procedure-4 .HPMS-Heading-3}

It is assumed that these highways operate as a multilane undivided
facility in the two-lane direction and as a single direction of a rural
two-lane facility in the one-lane direction. The two-way capacity
(required by HPMS for these facilities) is then the sum of these two
separate capacities.

The capacity for the two-lane direction is computed using the Multilane
Highway capacity procedure assuming an undivided cross-section. The
capacity for the one-lane direction is computed using the procedure for
Rural One-Lane, One-Way Highways.

Signalized Intersection Capacity

## Application {#application-5 .HPMS-Heading-3}

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 {#urban-procedure .HPMS-Heading-3}

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:

> $C_{A} = \sum_{i}^{}s_{i}\frac{g_{i}}{C}$ (20)

where:

*C~A~* = intersection approach capacity

*s~i~* = saturation flow rate for lane group *i*

= effective green ratio for lane group *i*

This approach is based directly on the *HCM 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   | **[Data   | **[No. Peak  | **[Lane    | **[No. Lanes in |
| Item      | Item      | Lanes]{.     | G          | Group           |
| 88]{.sma  | 89]{.sma  | smallcaps}** | roups]{.sm | ]{.smallcaps}** |
| llcaps}** | llcaps}** |              | allcaps}** |                 |
+===========+===========+==============+============+=================+
| 1         | 1         | N/A          | Exclusive  | 2               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 2               |
|           |           |              |            |                 |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 2, 3      |              | Exclusive  | 2               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 1               |
|           |           |              |            |                 |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 4         | 1, 2         | Exclusive  | 2               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Shared     |                 |
|           |           |              | Through/RT |                 |
+-----------+-----------+--------------+------------+-----------------+
|           |           | 3+           | Exclusive  | 2               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*- 1          |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 1               |
|           |           |              |            |                 |
|           |           |              | Shared     |                 |
|           |           |              | Through/RT |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 5         | N/A          | Exclusive  | 2               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    |                 |
+-----------+-----------+--------------+------------+-----------------+
| 2,3       | 1         | N/A          | Exclusive  | 1               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 2               |
|           |           |              |            |                 |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 2, 3      |              | Exclusive  | 1               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 1               |
|           |           |              |            |                 |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 4         | 1, 2         | Exclusive  | 1               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Shared     |                 |
|           |           |              | Through/RT |                 |
+-----------+-----------+--------------+------------+-----------------+
|           |           | 3+           | Exclusive  | 1               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*- 1          |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 1               |
|           |           |              |            |                 |
|           |           |              | Shared     |                 |
|           |           |              | Through/RT |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 5         | N/A          | Exclusive  | 1               |
|           |           |              | LT         |                 |
|           |           |              |            | *N*             |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    |                 |
+-----------+-----------+--------------+------------+-----------------+
| 4         | 1         | 1, 2         | Shared     | Data Item 87    |
|           |           |              | LT/Through |                 |
|           |           |              |            | 2               |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           |           | 3+           | Shared     | 1               |
|           |           |              | LT/Through |                 |
|           |           |              |            | *N*-- 1         |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 2               |
|           |           |              |            |                 |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 2, 3      | 1, 2         | Shared     | *N*             |
|           |           |              | LT/Through |                 |
|           |           |              |            | 1               |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           |           | 3+           | Shared     | 1               |
|           |           |              | LT/Through |                 |
|           |           |              |            | *N*-- 1         |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 1               |
|           |           |              |            |                 |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 4         | 1, 2         | Shared     | *N*             |
|           |           |              | LT/        |                 |
|           |           |              | Through/RT |                 |
+-----------+-----------+--------------+------------+-----------------+
|           |           | 3+           | Shared     | 1               |
|           |           |              | LT/Through |                 |
|           |           |              |            | *N*-- 2         |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    | 1               |
|           |           |              |            |                 |
|           |           |              | Shared     |                 |
|           |           |              | RT/Through |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 5         | 1, 2         | Shared     | *N*             |
|           |           |              | LT/Through |                 |
+-----------+-----------+--------------+------------+-----------------+
|           |           | 3+           | Shared     | 1               |
|           |           |              | LT/Through |                 |
|           |           |              |            | *N*-- 1         |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    |                 |
+-----------+-----------+--------------+------------+-----------------+
| 5         | 1         | N/A          | Exclusive  | *N*             |
|           |           |              | Through    |                 |
|           |           |              |            | 2               |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 2, 3      | N/A          | Exclusive  | *N*             |
|           |           |              | Through    |                 |
|           |           |              |            | 1               |
|           |           |              | Exclusive  |                 |
|           |           |              | RT         |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 4         | 1, 2         | Shared     | *N*             |
|           |           |              | RT/Through |                 |
+-----------+-----------+--------------+------------+-----------------+
|           |           | 3+           | Shared     | 1               |
|           |           |              | RT/Through |                 |
|           |           |              |            | *N*-- 1         |
|           |           |              | Exclusive  |                 |
|           |           |              | Through    |                 |
+-----------+-----------+--------------+------------+-----------------+
|           | 5         | N/A          | Exclusive  | *N*             |
|           |           |              | Through    |                 |
+-----------+-----------+--------------+------------+-----------------+

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:

> $s = s_{o}{Nf}_{w}f_{\text{HV}}f_{g}f_{p}f_{\text{bb}}f_{a}f_{\text{LU}}f_{\text{LT}}f_{\text{RT}}f_{\text{Lpb}}f_{\text{Rpb}}\text{PHF}$
> (21)

where:

> $s =$ saturation flow rate for subject lane group, expressed as a
> total for all lanes in lane group (vph);
>
> $s_{o} =$ base saturation flow rate per lane (pcphpl);
>
> $N =$ number of lanes in lane group;
>
> $f_{w} =$ adjustment factor for lane width;
>
> $f_{\text{HV}} =$ adjustment factor for heavy vehicles in traffic
> stream;
>
> $f_{g} =$ adjustment factor for approach grade;
>
> $f_{p} =$ adjustment factor for existence of a parking lane and
> parking activity adjacent to lane group;
>
> $f_{\text{bb}} =$ adjustment factor for blocking effect of local buses
> that stop within intersection area;
>
> $f_{a} =$ adjustment factor for area type;
>
> $f_{\text{LU}} =$ adjustment factor for lane utilization;
>
> $f_{\text{LT}} =$ adjustment factor for left turns in lane group;
>
> $f_{\text{RT}} =$ adjustment factor for right turns in lane group;
>
> $f_{\text{Lpb}} =$ pedestrian-bicycle adjustment factor for left-turn
> movements;
>
> $f_{\text{Rpb}} =$ pedestrian-bicycle adjustment factor for right-turn
> movements; and
>
> $\text{PHF} =$ 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, *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:

> $f_{W} = 1 + \frac{(W - \text{12})}{\text{30}}$ (22)

where:

> $W = \text{ Lane Width}$(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:

> $f_{\text{HV}} = \frac{\text{100}}{\text{100} + \text{HV}(E_{T} - 1)}$
> (23)

where:

*HV* = percent heavy vehicles

*E~T~* = 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, *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:

> $f_{p} = \frac{N - 0\text{.}1 - \frac{\text{18}N_{m}}{3,\text{600}}}{N}$
> (24)

where:

*f~p~* = parking adjustment factor

*N* = number of lanes in group

*N~m~* = 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), *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    | **F       | ***f~LT~***  |
|                               | Item 88** | unctional |              |
|                               |           | Class**   |              |
+-------------------------------+-----------+-----------+--------------+
| 1.  Protected Phasing,        | 1, 2, 3   | All       | $$           |
|     Exclusive LT              |           | A         | f_{\text{LT} |
|                               |           | rterials; | } = 0\text{. |
|                               |           |           | }\text{95}$$ |
|                               |           | Rural     |              |
|                               |           | Major     |              |
|                               |           | C         |              |
|                               |           | ollectors |              |
+-------------------------------+-----------+-----------+--------------+
| 2.  Permitted Phasing,        | 1, 2, 3   | Rural     | (see below)  |
|     Exclusive LT              |           | Minor     |              |
|                               |           | Co        |              |
|                               |           | llectors; |              |
|                               |           | Urban     |              |
|                               |           | C         |              |
|                               |           | ollectors |              |
+-------------------------------+-----------+-----------+--------------+
| 3.  Protected/Permitted       | N/A       | N/A       | N/A          |
|     Phasing, Exclusive LT     |           |           |              |
+-------------------------------+-----------+-----------+--------------+
| 4.  Protected Phasing, Shared | 4         | Principal | $$           |
|     LT                        |           | Arterials | f_{\text{LT} |
|                               |           |           | } = \frac{1} |
|                               |           |           | {1\text{.}0  |
|                               |           |           | + 0\text{.}\ |
|                               |           |           | text{05}P_{\ |
|                               |           |           | text{LT}}}$$ |
|                               |           |           |              |
|                               |           |           | *(P~LT~* =   |
|                               |           |           | 0.10)        |
+-------------------------------+-----------+-----------+--------------+
| 5.  Permitted Phasing, Shared | 4         | Minor     | (see below)  |
|     LT                        |           | A         |              |
|                               |           | rterials; |              |
|                               |           | All       |              |
|                               |           | C         |              |
|                               |           | ollectors |              |
+-------------------------------+-----------+-----------+--------------+
| 6.  Protected/Permitted       | N/A       | N/A       | N/A          |
|     Phasing, Shared LT        |           |           |              |
+-------------------------------+-----------+-----------+--------------+

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:

*f~LT~* = (1400--*V~o~*)/\[(1400--*V~o~*)+(235+0.435\**V~o~*)\**P~LT~*\]
for *V~o\ ~*\<= 1,220 vph (25)

= 1/(1+(4.525\**P~LT~*)) for *V*~o~ \>1,220 vph

Where: *V~o~* = 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

*P~LT\ ~* = proportion of left turns; assume proportion of left turns is
10 percent

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:

*f~RT~* = 0.85 (for Data Item 89 = 1, 2, or 3) (26)

= 1.0 -- 0.15*P~RT\ ~*(for Data Item 89 = 4)

where:

> $$\begin{array}{r}
> P_{\text{RT}} = \text{ proportion of right turns in the lane group} \\
> 0\text{.}\text{10 }(\text{HCM, page 10-19})
> \end{array}$$

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

*g~LT~ /C* = 0.35\**g/C* for principal arterials (27)

= 0.25\**g/C* for all other highway types

For *g/C* \> 0.65

*g~LT~ /C* = 0.5\*(1 - *g/C*) (all highways) (28)

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).

Simplified Procedure For Rural Signalized Intersection Capacity And
Urban Sections With Missing Data

## Application {#application-6 .HPMS-Heading-3}

In the coding of HPMS data, several data items required for applying the
urban signalized capacity analysis are either "optional" or not required
for rural sections. The data items of interest for capacity
calculations, since they are used to identify lane groups, are:

-   Data Item 88: Left Turning Lanes

-   Data Item 89: Right Turning Lanes.

## Procedure {#procedure-5 .HPMS-Heading-3}

Although some states do code these items for rural sections, a provision
must be made to handle cases where the data are not present; this could
also be true for some urban sections. In the cases where rural
signalized sections have nonzero values coded for these data items, the
signalized intersection capacity is used. When these data are coded as
zero, the following procedure is used:

*C~A~* = 1,900 \* *N* \* *f~w~* \* *f~HV~* \* *PHF* \* *g/C* (29)

where:

*C~A~* = intersection approach capacity

*N* = number of lanes on the segment (one direction)

= Data Item 87 (Number of Peak Lanes)

*f~w~* = adjustment factor for lane width (use Equation 22)

*f~HV~* = adjustment factor for heavy vehicles (use Equation 23)

*PHF* = Peak Hour Factor

= 0.88 for rural conditions, 0.92 for urban conditions

*g/C* = effective green time-to-cycle length ratio

= 0.55 for principal arterials

= 0.45 for minor arterials

= 0.40 for collectors

The *g/C* ratio default values given above attempt to account for, in a
general way, the presence of exclusive turn lanes and phases. These
values are higher than the default values listed in Table 9-3 of *NCHRP
387* (0.40-0.45) but those values consider only through movement
capacity.

Stop Sign-Controlled Highway Capacity

## Application {#application-7 .HPMS-Heading-3}

All highways that have at least one stop sign and meet the density
requirement (\> .5/mile) on the inventory route.

## Procedure {#procedure-6 .HPMS-Heading-3}

The *HCM 2000* procedure for stop sign-controlled intersections is
complex and highly dependent on turning movements on all the approaches.
Procedures also vary for two-way controlled intersections versus all-way
controlled intersections. None of these data are present in HPMS.
Therefore, a highly simplified method is recommended. It is based on the
*HCM 2000* procedure for two-way controlled intersections. This is the
simpler and less data intensive of the two methods.

The procedure is based on first calculating the potential capacity for
each turning movement. (It is assumed that the potential capacity is
equal to the "movement capacity", and none of the *HCM 2000* procedures
for converting potential capacity to movement capacity are used.) Where
shared lanes exist for turning movements, an extra calculation is
required to derive the potential (movement) capacity of the shared lane.
This step combines the potential capacities for two or three turning
movements (depending on the lane configuration) into a single potential
capacity. The final capacity of the stop sign-controlled approach is
then the sum of all the potential capacities calculated for the turning
movements.

Step 1: Calculate Potential Capacity of Each Turning Movement

The procedure starts with *HCM* Equation 17-3. It first calculates the
potential capacity for each of the movements on the stop sign-controlled
approach:

$C_{p,x} = \text{CV}_{c,x}\frac{e^{- V_{c,x}t_{c,x}/\text{3600}}}{1 - e^{- V_{c,x}t_{f,x}/\text{3600}}}$
(30)

Where:

*C~p,x~ =* potential capacity of movement *x* (vph)

*CV~c,x~ =* conflicting flow rate for each movement *x* (vph); Table 12

*t~c,x~ =* critical gap (seconds) for each movement *x*

= *t~c,base~ + ( P~HV\ ~*\* *t~c,HV~*)

*t~c,base\ ~*= default values from Table 11

*t~c,HV\ ~*= 1.0 for one or two-through lane roads (Data Item 34 \<= 2)

= 2.0 otherwise

*P~HV\ ~*= percent of heavy vehicles in traffic stream, peak period,
expressed as a decimal

*t~f,x~ =* follow-up time (seconds) for each movement *x*

= *t~f,base~ + ( P~HV\ ~*\* *t~f,HV~*)

*t~f,HV\ ~*= 0.9 for one or two-through lane roads (Data Item 34 \<= 2)

= 1.0 otherwise

Table 11. Default Values for Calculating Potential Capacities (*C~p,x~*)
of Stop Sign-Controlled Highways

+-----------------------+-----------------------+-----------------------+
| ####### **Veh         | **Base Critical Gap,  | **Follow-up Time,     |
| icle Movement (*x*)** | *t~c,base~***         | *t~f,base~***         |
+-----------------------+-----------------------+-----------------------+
| Right Turns           | 6.2                   | 3.3                   |
+-----------------------+-----------------------+-----------------------+
| Through               | 6.5                   | 4.0                   |
+-----------------------+-----------------------+-----------------------+
| Left Turns            | 7.1                   | 3.5                   |
+-----------------------+-----------------------+-----------------------+

Table 12. Default Values for Conflicting Flow Rates

+-----------------------------------+-----------------------------------+
| **Functional Class**              | **Conflicting Flow**              |
|                                   |                                   |
|                                   | **Rate, *CV~c,x~***               |
+-----------------------------------+-----------------------------------+
| Rural Principal Arterials         | 100                               |
+-----------------------------------+-----------------------------------+
| Rural Minor Arterials             | 150                               |
+-----------------------------------+-----------------------------------+
| Other Rural                       | 200                               |
+-----------------------------------+-----------------------------------+
| Urban Principal Arterials         | 250                               |
+-----------------------------------+-----------------------------------+
| Urban Minor Arterials             | 500                               |
+-----------------------------------+-----------------------------------+
| Other Urban                       | 750                               |
+-----------------------------------+-----------------------------------+

Clearly, the major assumption in this Step is that of the conflicting
volumes. Basically, it is assumed that the intersection is composed of a
higher class facility in one direction and a lower class facility in the
other. Many stop sign-controlled intersections, particularly those with
all-way control, are comprised of similar class facilities intersecting.
However, without detailed knowledge of such occurrences, the assumption
is a reasonable default.

Step 2: Determine Approach Capacity

Several adjustments to potential capacity are defined in the *HCM 2000*
in order to derive "movement capacity". Most of the adjustments are
based on knowledge of turning movements on and the geometry of all
approaches of the intersection. Since none of these data are available
in HPMS, major assumptions would be required. ***As an alternative, the
movement capacity is set equal to potential capacity for most cases,
except for shared lanes.*** The adjustment for shared-lane capacity is
essential for calculating intersection approach capacity. *HCM* Equation
17-15 is used in these cases:

$C_{p,\text{SH}} = \frac{\sum_{x}^{}V_{x}}{\sum_{x}^{}\left( \frac{V_{x}}{C_{p,x}} \right)}$
(31)

Where:

*C~p,SH~ =* potential capacity of the shared lane (vph)

*V~x~ =* flow rate of the *x* movement in the shared lane (vph)

*C~p,x~ =* potential capacity of the *x* movement in the shared lane
(vph) from Equation 1.

Volumes for the turning movements on the intersection approach in
question must be made in order to apply Equation 30. It is assumed that
10 percent of the approach volume is composed of left-turning vehicles
and 10 percent composed of right turning vehicles (recommended defaults
from Chapter 10 of *HCM 2000*). Total approach volume is calculated as:

Approach Volume = AADT \* (K \* 0.01) \* (D \* 0.01) (32)

Where:

AADT = Data Item 33

K = Data Item 85

D = Data Item 86

= MIN(70, Data Item 86)

Values for V~x~ are then computed for left turns, through movements and
right turns by applying the assumed turning percentages to the approach
volume: 10%, 80%, and 10%, respectively.

The number of lanes handling turning movements on the approach are then
determined. This is based on Data Items 88 and 89 (Left and Right
Turning Lanes) for urban sections and for rural sections where these
data are coded. Table 3 shows how to handle each of the possible cases.
The approach capacity is based on Equation 30 for nonshared lanes and
Equation 31 for shared lanes. In some cases both terms are used. Note
that *C~A~* is adjusted to account for the number of lanes handling each
movement. Where HPMS indicates "multiple" turning lanes, it is assumed
that two such lanes exist. ***For rural sections -- or for any sections
where Data Items 88 and 89 are coded as zero -- it is assumed that no
exclusive left turn or right turn lanes exist.***

Table 13. Determining Stop Sign-Controlled Intersection Approach
Capacity (*C~A~*)

+-----------------+------------------+---------------------------------+
| **Left Turning  | **Right Turning  | **Approach Capacity             |
| Bay/Lanes       | Bay/Lanes Coding | Calculation, *C~A\ ~*(vph)**    |
| Coding (Data    | (Data Item 89)** |                                 |
| Item 88)**      |                  |                                 |
+-----------------+------------------+---------------------------------+
|                 | 0, 4             | All movements made from a       |
|                 |                  | shared lane;                    |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~T~C~p,SH~*            |
+-----------------+------------------+---------------------------------+
| 0, 4            | 1, 2, 3          | Shared LT + T lane; exclusive   |
|                 |                  | RT lane;                        |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~T~ C ~p,SH(LT+T)~ +   |
|                 |                  | N~RT~C~p,RT~*                   |
+-----------------+------------------+---------------------------------+
|                 | 5                | Do not consider right turn      |
|                 |                  | volumes;                        |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~T~C ~p,SH(LT+T)~*     |
+-----------------+------------------+---------------------------------+
|                 | 0, 4             | Shared RT + T lane; exclusive   |
|                 |                  | LT lane                         |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~T~C ~p,SH(RT+T)~ +    |
|                 |                  | N~LT~C~p,LT~*                   |
+-----------------+------------------+---------------------------------+
| 1, 2, 3         | 1, 2, 3          | Exclusive lanes for all         |
|                 |                  | movements;                      |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~LT~C~p,LT~ +          |
|                 |                  | N~T~C~p,T~ + N~RT~C~p,RT~*      |
+-----------------+------------------+---------------------------------+
|                 | 5                | Do not consider right turn      |
|                 |                  | volumes;                        |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~LT~C~p,LT~ +          |
|                 |                  | N~T~C~p,T~*                     |
+-----------------+------------------+---------------------------------+
|                 | 0, 4             | Do not consider left turn       |
|                 |                  | volumes                         |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~T~C ~p,SH(RT+T)~*     |
+-----------------+------------------+---------------------------------+
| 5               | 1, 2, 3          | Do not consider left turn       |
|                 |                  | volumes                         |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~T~C~p,T~ +            |
|                 |                  | N~RT~C~p,RT~*                   |
+-----------------+------------------+---------------------------------+
|                 | 5                | Consider only through volumes   |
|                 |                  |                                 |
|                 |                  | *C~A~ = N~T~C~p,T~*             |
+-----------------+------------------+---------------------------------+

Where:

*N~T~* = Number of Peak Through Lanes

= 1 for rural highways with 2 through lanes

= 2 for rural highways with 3 through lanes

= Data Item 87 otherwise

*N~LT\ ~*= Number of Left Turn Lanes

= 2 if Data Item 88 = 1

= 1 if Data Item 88 = (2,3)

= 0 otherwise

*N~RT~* = Number of Right Turn Lanes

= 2 if Data Item 89 = 1

= 1 if Data Item 89 = (2,3)

= 0 otherwise

*C~p,SH\ ~* = potential capacity of the shared lane (vph) from Equation
31 (the terms in parentheses indicate which movements are being handled
by the shared lane, e.g., "LT+T" means that left turns and through
movements share the lanes

*C~p,T\ ~*= potential capacity (assumed to be the movement capacity from
Equation 30) for the through movement

*C~p,RT\ ~* = potential capacity (assumed to be the movement capacity
from Equation 30) for the right turn movement

*C~p,LT\ ~* = potential capacity (assumed to be the movement capacity
from Equation 30) for the left turn movement

Step 3: Set Peak Capacity to Approach Capacity

For HPMS, the Peak Capacity of the section is then based on the approach
capacity of the stop sign-controlled intersection analyzed above. For
urban sections, since the HPMS definition of urban capacity is one-way,
the Peak Capacity is set equal to the approach capacity. For rural
sections, it is based on the total number of through lanes (Data Item
34). This is done to account for the definition of capacity for 2- and
3-lane sections in HPMS, which is two-way capacity.

Rural 4+ through lanes

Peak Capacity = Approach Capacity

Rural 3-through lanes

Peak Capacity = Approach Capacity \* 1.67 (assumes that the direction
with two lanes is the inventory direction)

Rural 2-through lanes

Peak Capacity = Approach Capacity \* 2

Urban One/Two/Three Lane Highway Capacity

## Application {#application-8 .HPMS-Heading-3}

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 {#procedure-7 .HPMS-Heading-3}

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

$\text{Capacity} = s_{o}{Nf}_{w}f_{\text{HV}}f_{p}f_{a}\text{PHF}$ (33)

where:

*s~o~* = base saturation flow rate per lane (pcphpl);

*N* = number of peak lanes (Data Item 87);

*f~w~* = adjustment factor for lane width;

*f~HV~* = adjustment factor for heavy vehicles in traffic stream;

*f~p~* = adjustment factor for existence of parking activity;

*f~a~* = adjustment factor for area type;

*PFH* = Peak Hour Factor.

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:

$f_{W} = 1 + \frac{(W - \text{12})}{\text{30}}$ (34)

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:

$f_{\text{HV}} = \frac{\text{100}}{\text{100} + \text{HV}(E_{T} - 1)}$
(35)

where:

*HV* = percent heavy vehicles

*E~T~ =* 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, *f~p~*

The calculation of the parking adjustment factor is:

$f_{p} = \frac{N - 0\text{.}1 - \frac{\text{18}N_{m}}{3,\text{600}}}{N}$
(36)

where:

*f~p~* = parking adjustment factor

*N* = number of lanes in lane group

*N~m~* = 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), *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

  -------------------------------------------------------------------------------
  **Item     **Description**            **Item     **Description**
  Number**                              Number**   
  ---------- -------------------------- ---------- ------------------------------
  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         Directional 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      94         Number At-Grade
             Distance - **Rural**                  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
  -------------------------- --------------------------------------------

[^1]: Two-Way Left Turning Lanes.