# CHAPTER 5: CAPACITY AND PERFORMANCE ANALYSIS

## 5.1 Introduction

The next step of this project was to determine the capacity deficiencies of the freight transportation highway network based on the supply and demand of freight (truck traffic) flows. Given that truck travel patterns are different from commuter travel patterns, it is necessary to determine the effects of truck traffic on capacity requirements of the network for truck peak hour design flows.

## 5.2 Capacity Analysis

The highway capacity analysis was intended to provide information on a set of performance measures for each highway link. Highway capacity-related performance measures include traffic volume, travel time, link delay, congested speed, and service to flow ratio. These performance measures were estimated for the 2007 base year as well as the forecast year 2040. Differences in these performance measures between the base year and the forecast year are indications of changes in congestion and the ability of the highway system capacity to support freight transportation system demand in the future.

The capacity and performance results can be used to generate thematic maps that show highway links with capacity problems. The performance measures can also be aggregated to identify those congested highway links connecting border crossings, seaports, airports, and other gateways including intermodal transfer points. The performance measures resulting from the analysis can also be organized and aggregated to serve as inputs to other policy-related analyses.

This section explains the calculation of the performance measures for capacity analysis including

- Traffic Volume
- Design Hour Volume
- Capacity
*v/c*(volume:capacity) ratios- Travel times, Speed, and Delay.

### 5.2.1 Traffic Volume

The traffic volume on any link on the network, for a particular forecast year *x*, is the sum of passenger vehicles, freight trucks, and non-freight trucks as expressed in Equation 5-1.

*V*(5-1)

_{x}= N_{x}+ FAF_{x}+ P_{x}where

** V_{x}** = The total AADT volume in a segment of the network

**= The total non-FAF trucks on the segment**

*N*_{x}**= The total freight trucks on the segment**

*FAF*_{x}**= The passenger cars on the segment.**

*P*_{x}The freight truck volumes on each highway link for 2007 and 2040 are outputs of the assignment process. This relationship holds true for both years. However, the calculation of non-freight and passenger volumes varies from 2007 to 2040. The non-FAF truck volume in 2007 was calculated as follows:

where ** AADTT** = baseline HPMS truck volume for 2007.

The passenger car volume is the difference between the AADT and the AADTTs available for each segment.

*P _{2007} = AADT_{2007} – AADTT_{2007}* (5-3)

where ** AADT** = baseline total traffic volumes for 2007.

In order to obtain the corresponding passenger and non-FAF truck traffic volumes for 2040, growth factors were applied. The growth factors were derived using the county-specific HPMS growth factor using HPMS vehicle miles traveled (VMT, for sample sections only) over the next 20 years from the base year 2007. A straight-line projection was used to estimate the 2040 growth rate. The volumes of passenger cars and non-freight trucks for 2040 is given by equations 6.4 and 6.5 as follows.

*P _{2040} = P_{2007}* (1 +

*G*)

_{p}^{33}(5-4)

*N _{2040} = N_{2007}* (1 +

*G*)

_{t}^{33}(5-5)

Where

** G_{p}**= HPMS growth rate for passenger cars per year

**= HPMS growth rate for trucks per year**

*G*_{t}*P*= passenger car traffic volume for 2007

_{2007}*P*= passenger car traffic volume for 2040

_{2040}*N*= non-FAF truck traffic volume for 2007

_{2007}*N*= non-FAF truck traffic volume for 2040.

_{2040}### 5.2.2 Design Hour Volume

While daily volumes are useful for planning purposes, they cannot be used alone for design or operational analysis. Volume varies considerably during the course of the day and by direction. The peak hour volumes are often used as the basis for highway design and for many types of operational analysis. One way of estimating peak hourly volumes is to use the daily volume projections and the K factor, using the following relationship.

DHVx = V_{x x} K-Factor (5-6)

where

DHV_{x} = Design hour volume on the link for forecast year *x (vph)*

V_{x} = Daily volume on the link for forecast year *x (vpd)*

K-Factor = Proportion of daily traffic occurring during the peak hour, expressed as a decimal. For design purposes, this represents the proportion of AADT occurring during the 30^{th} highest peak hour of the year.

The AADT volumes for 2007 and 2040 were converted using the above relationships to obtain design hour volumes.

### 5.2.3 Capacity

The capacity of a segment is assumed as the service volume at LOS E. The general procedures for estimating highway capacity for 2-lane facilities, multilane facilities—divided and undivided, freeways by design are included in Appendix N of the HPMS Manual and utilize Highway Capacity Manual (HCM) 2000 procedures.

### 5.2.4 V/C Ratios

A critical factor in any capacity analysis is the proportion of the facility’s capacity being utilized by current or projected traffic. This ratio is often used as a measure of sufficiency of existing or proposed capacity. In forecasting situations, a volume to capacity (v/c) ratio above 1.00 predicts that the facility will fail, i.e., be unable to discharge the demand arriving at the section, leading to excessive delays and queues. The v/c ratios for each segment are calculated by dividing the design hour volume by the capacity calculated in the previous section. These ratios are then classified into three groups: <0.75, 0.75 to 95.0, and >0.95 for different groupings like functional classes, rural/urban, etc. The v/c ratios can also be combined with the network and plotted thematically, allowing visual inspection of congested segments of the roadway.

### 5.2.5 Travel Time, Speed, and Delay

The congested travel time and link speed for the FAF network was estimated using the HCM 2000 area wide planning level procedure listed in Chapter 30 of the HCM 2000. The vehicle speed for the FAF link was computed using Equation 5-7:

Where

S= link speed

L=link length

R=link transversal time (h)

D = Node delay and assumed zero for this exercise.

The link traversal time was calculated using Equation 5-8:

Where

R = link transversal time (h)

R_{0} = link transversal time at link FFS (h)

D_{0} = zero-flow control delay at signalized intersection (h)

T = expected duration of demand (typically 1h) (h)

X = link demand to capacity ratio

J = calibration parameter

L = link length (mi).

The link transversal time for free-flow condition (R_{0}) was computed from the FFS, using Equation 5-9:

Where

R_{0} = link transversal time at link FFS (h)

L = link length (mi)

S_{0} = link FFS (mi/h).

The calibration parameter J was selected so that the transversal time equation will predict the mean speed of the traffic when demand is equal to capacity. Substituting x = 1.00 in the transversal time equation and solving for J yields the equation:

Where

J = calibration parameter

R_{0} = link transversal time at link FFS (h)

R_{0} = link transversal time when demand equals capacity (h)

L = link length (mi).

The assumed values of parameter J for freeway, multilane, and urban links are listed in Table 5-1 and are derived from HCM Exhibit 30-4. Table 5-2 lists the J values for urban links. Since FAF used the BPR vehicle delay function for freight assignment, these values are more appropriate for estimating link transversal times.

RoadType | MinSpeed | MaxSpeed | J |
---|---|---|---|

1 | 0 | 55 | 3.31E-06 |

1 | 56 | 60 | 8.65E-06 |

1 | 61 | 65 | 0.0000148 |

1 | 66 | 70 | 0.000021 |

1 | 71 | 85 | 0.0000269 |

2 | 0 | 45 | 2.52E-06 |

2 | 46 | 50 | 1.63E-06 |

2 | 51 | 55 | 2.03E-06 |

2 | 56 | 80 | 0.0000023 |

3 | 0 | 44 | 0.000929 |

3 | 45 | 50 | 0.0004 |

3 | 51 | 56 | 0.000202 |

3 | 57 | 63 | 0.000114 |

3 | 64 | 80 | 0.0000691 |

RoadType | FClass | J |
---|---|---|

4 | 14 | 0.000468 |

4 | 16 | 0.000502 |

4 | 17 | 0.00455 |

4 | 19 | 0.0137 |

## 5.3 Highway Capacity Impacts

A summary of highway capacity impacts due to 2007 and 2040 freight truck traffic volumes was also estimated, using the service v/c ratio value. The impact on highway capacity is expressed as the miles of highway that fall into one of three categories based on the following v/c ratios:

- Below capacity – v/c less than 0.75
- Approaching capacity – v/c ratio 0.75 to 0.95
- Exceeding capacity – v/c ratio greater than 0.95.

Figure 5-1 and Figure 5-2 illustrate highway congestion for 2007 and 2040.

Source: U.S. Department of Transportation, Federal Highway Administration, Office of Highway Policy Information, Highway Performance Monitoring System, and Office of Freight Management and Operations, Freight Analysis Framework, version 3.1, 2010.

**Figure 5-1. NHS Highway Network Congestion for Year 2007**

Source: U.S. Department of Transportation, Federal Highway Administration, Office of Freight Management and Operations, Freight Analysis Framework, version 3.1, 2010.

**Figure 5-2. NHS Highway Network Congestion for Year 2035**

The capacity analysis results for the scenario analysis are summarized in Table 5-3, which shows the highway miles in the three categories of capacity as defined by the v/c ratios by functional highway class. Highway miles in each v/c category are also expressed as percentages of the total highway miles in that functional class and state.

Miles of Highway by V/C Ratio Category | Miles of Highway by V/C Ratio Category | Miles of Highway by V/C Ratio Category | 2007 | 2007 | 2040 | 2040 |
---|---|---|---|---|---|---|

NHS,IS (miles) | Urban | <= 0.75 | 8,565 | 51.96% | 2,785 | 16.90% |

NHS,IS (miles) | Urban | 0.75 - < 0.95 | 4,558 | 27.65% | 2,299 | 13.95% |

NHS,IS (miles) | Urban | > 0.95 | 3,361 | 20.39% | 11,399 | 69.16% |

NHS,IS (miles) | Rural | <= 0.75 | 27,361 | 90.65% | 15,640 | 51.82% |

NHS,IS (miles) | Rural | 0.75 - < 0.95 | 2,239 | 7.42% | 5,279 | 17.49% |

NHS,IS (miles) | Rural | > 0.95 | 583 | 1.93% | 9,265 | 30.70% |

NHS, Non-IS (miles) | Urban | <= 0.75 | 27,495 | 82.57% | 16,283 | 48.90% |

NHS, Non-IS (miles) | Urban | 0.75 - < 0.95 | 3,406 | 10.23% | 6,095 | 18.30% |

NHS, Non-IS (miles) | Urban | > 0.95 | 2,397 | 7.20% | 10,920 | 32.79% |

NHS, Non-IS (miles) | Rural | <= 0.75 | 82,207 | 97.74% | 70,053 | 83.29% |

NHS, Non-IS (miles) | Rural | 0.75 - < 0.95 | 1,416 | 1.68% | 6,486 | 7.71% |

NHS, Non-IS (miles) | Rural | > 0.95 | 483 | 0.57% | 7,568 | 9.00% |

All NHS (miles) | All NHS (miles) | <= 0.75 | 145,628 | 88.76% | 104,761 | 63.85% |

All NHS (miles) | All NHS (miles) | 0.75 - < 0.95 | 11,619 | 7.08% | 20,159 | 12.29% |

All NHS (miles) | All NHS (miles) | > 0.95 | 6,824 | 4.16% | 39,152 | 23.86% |

Non-NHS (miles) | Urban | <= 0.75 | 55,795 | 93.14% | 37,747 | 63.01% |

Non-NHS (miles) | Urban | 0.75 - < 0.95 | 2,463 | 4.11% | 9,595 | 16.02% |

Non-NHS (miles) | Urban | . 0.95 | 1,647 | 2.75% | 12,562 | 20.97% |

Non-NHS (miles) | Rural | <= 0.75 | 217,698 | 99.09% | 200,035 | 91.05% |

Non-NHS (miles) | Rural | 0.75 - < 0.95 | 1,283 | 0.58% | 9,550 | 4.35% |

Non-NHS (miles) | Rural | > 0.95 | 714 | 0.32% | 10,110 | 4.60% |

All FAF network (miles) | All FAF network (miles) | <= 0.75 | 419,121 | 94.47% | 342,543 | 77.21% |

All FAF network (miles) | All FAF network (miles) | 0.75 - < 0.95 | 15,365 | 3.46% | 39,304 | 8.86% |

All FAF network (miles) | All FAF network (miles) | > 0.95 | 9,185 | 2.07% | 61,824 | 13.93% |

**NOTE:** The analysis excludes ferry and roadway through Canada.

Figure 5-3 shows the number of interstate and non-interstate highway miles that exceed capacity in the years 2007 and 2040. Figure 5-4 shows the percentage of miles exceeding the capacity in the year 2007 and 2040.

- In all, 4.16 percent of NHS miles exceeded the capacity in 2007, and this will increase to 23.86 percent in 2040.
- In 2007, 483 miles of rural interstate exceeded the capacity, and the miles with heavy congestion will increase to 9,265 miles in 2040, which represents 30.70 percent of the total rural interstate miles.
- About 3,361 miles of urban interstate were heavily congested in 2007, accounting for 20.39 percent of the total NHS urban interstate miles. This percentage increases considerably to 69.16 percent in 2040, corresponding to 11,399 miles of urban interstate that will exceed the capacity.

**Figure 5-3. NHS Highway Miles Exceeding Capacity**

**Figure 5-4. Percentage of NHS Highway Miles Exceeding Capacity**

Figure 5-5 shows miles of NHS interstate for the year 2007 and 2040 by volume to capacity ratio.

**Figure 5-5. NHS Interstate Highway Miles by Capacity Categories**

## 5.4 Highway Impact on Truck Traffic

The summary statistics for the years 2007 and 2040 are shown in Table 5-4 based on the truck traffic for various highway functional classes. The table represents the highway miles based on the functional class and AADTT volume groups. The three truck volume groups used are:

- Light Truck Traffic – 0 – 5,000 AADTT
- Moderate Truck Traffic – 5,000 to 10,000 AADTT
- Heavy Truck Traffic – greater than 10,000 AADTT.

Figures 5-6 to 5-8 show the interstate, NHS non-interstate, and non-NHS miles by truck volume groups.

- In all, 80.38 percent and 65.10 percent of the NHS miles in the years 2007 and 2040 respectively carry less than 5,000 AADTT.
- In 2007, 6,360 (38.63 percent) miles of urban NHS interstate and 3,479 miles (11.53 percent) of rural NHS interstate carry more than 10,000 trucks per day. The miles with heavy truck traffic will increase more than twofold in 2040 to 12,615 miles (76.52 percent) of urban NHS interstate and 19,261 miles (63.81 percent) of rural NHS interstate.
- Out of the total 163,852 NHS miles, only 10,879 miles (6.64 percent) in 2007 and 36,045 miles (21.97 percent) in 2040 experience heavy truck traffic.
- The table shows that 0 percent to 2 percent of non-NHS miles carry daily truck traffic of more than 10,000 AADTT in 2007 and 2040. The percentage of non-NHS miles with heavy truck traffic is comparatively lower than rest of the highways in the NHS network.

Miles of Highway AADTT Volume Groups | Miles of Highway AADTT Volume Groups | Miles of Highway AADTT Volume Groups | 2007 | 2007 | 2040 | 2040 |
---|---|---|---|---|---|---|

NHS,IS (miles) | Urban | 0-5,000 | 3,536 | 21.48% | 1,145 | 6.94% |

NHS,IS (miles) | Urban | 5,000-10,000 | 6,568 | 39.90% | 2,726 | 16.53% |

NHS,IS (miles) | Urban | > 10,000 | 6,360 | 38.63% | 12,615 | 76.52% |

NHS,IS (miles) | Rural | 0-5,000 | 15,571 | 51.59% | 4,320 | 14.31% |

NHS,IS (miles) | Rural | 5,000-10,000 | 11,135 | 36.89% | 6,604 | 21.88% |

NHS,IS (miles) | Rural | > 10,000 | 3,479 | 11.53% | 19,261 | 63.81% |

NHS, Non-IS (miles) | Urban | 0-5,000 | 29,589 | 89.18% | 24,535 | 73.64% |

NHS, Non-IS (miles) | Urban | 5,000-10,000 | 2,688 | 8.10% | 5,850 | 17.56% |

NHS, Non-IS (miles) | Urban | > 10,000 | 902 | 2.72% | 2,932 | 8.80% |

NHS, Non-IS (miles) | Rural | 0-5,000 | 83,014 | 98.80% | 76,827 | 91.34% |

NHS, Non-IS (miles) | Rural | 5,000-10,000 | 872 | 1.04% | 6,044 | 7.19% |

NHS, Non-IS (miles) | Rural | > 10,000 | 138 | 0.16% | 1,237 | 1.47% |

All NHS (miles) | All NHS (miles) | 0-5,000 | 131,710 | 80.38% | 106,826 | 65.10% |

All NHS (miles) | All NHS (miles) | 5,000-10,000 | 21,263 | 12.98% | 21,224 | 12.93% |

All NHS (miles) | All NHS (miles) | > 10,000 | 10,879 | 6.64% | 36,045 | 21.97% |

Non-NHS (miles) | Urban | 0-5,000 | 59,255 | 99.08% | 56,425 | 94.19% |

Non-NHS (miles) | Urban | 5,000-10,000 | 519 | 0.87% | 2,865 | 4.78% |

Non-NHS (miles) | Urban | > 10,000 | 30 | 0.05% | 617 | 1.03% |

Non-NHS (miles) | Rural | 0-5,000 | 219,508 | 99.98% | 218,262 | 99.35% |

Non-NHS (miles) | Rural | 5,000-10,000 | 32 | 0.01% | 1,357 | 0.62% |

Non-NHS (miles) | Rural | > 10,000 | 2 | 0.00% | 77 | 0.04% |

All FAF network (miles) | All FAF network (miles) | 0-5,000 | 410,474 | 92.62% | 381,513 | 85.98% |

All FAF network (miles) | All FAF network (miles) | 5,000-10,000 | 21,814 | 4.92% | 25,446 | 5.74% |

All FAF network (miles) | All FAF network (miles) | > 10,000 | 10,911 | 2.46% | 36,740 | 8.28% |

**NOTE:** The analysis excludes ferry and roadway through Canada.

**Figure 5-6. NHS Miles of Roadway by Truck Traffic (AADTT) Groups**

**Figure 5-7. Interstate Miles of Roadway by Truck Traffic (AADTT) Groups**

**Figure 5-8. NHS Non-Interstate Miles of Roadway by Truck Traffic (AADTT) Groups**