TEMPORARY LOSSES OF HIGHWAY CAPACITY AND IMPACTS ON PERFORMANCE: PHASE 2

Title | Contents | Acknowledgements | Exec. Summary
1. Intro | 2. Approach | 3. Crashes | 4. Breakdowns | 5. Work Zones | 6. Weather | 7. Signal Timing
8. RR Crossings | 9. Toll Facilities | 10. PUD | 11. Results Summary | 12. Next Steps | 13. References

1. Introduction

1.1  Background

Traffic congestion and its impacts significantly affect the nation's economic performance and the public's quality of life.  In most urban areas, travel demand routinely exceeds highway capacity during peak periods.  In addition, events such as crashes, vehicle breakdowns, work zones, adverse weather, railroad crossings, commercial truck pickup and delivery (PUD) activities in urban areas, and other factors such as toll collection facilities and sub-optimal signal timing cause temporary capacity losses, often worsening the conditions on already congested highway networks.  The impacts of these temporary capacity losses include delay, reduced mobility, and reduced reliability of the highway system.  They can also cause drivers to re-route or re-schedule trips. Such information is vital to formulating sound public policies for the highway infrastructure and its operation.

In response to this need, Oak Ridge National Laboratory, sponsored by the Federal Highway Administration (FHWA), made an initial attempt to provide nationwide estimates of the capacity losses and delay caused by temporary capacity-reducing events (Chin et al. 2002). This study, called the Temporary Loss of Capacity (TLC) study, estimated capacity loss and delay on freeways and principal arterials resulting from fatal and non-fatal crashes, vehicle breakdowns, and adverse weather, including snow, ice, and fog. In addition, it estimated capacity loss and delay caused by sub-optimal signal timing at intersections on principal arterials. It also included rough estimates of capacity loss and delay on Interstates due to highway construction and maintenance work zones. Capacity loss and delay were estimated for calendar year 1999, except for work zone estimates, which were estimated for May 2001 to May 2002 due to data availability limitations. Prior to the first phase of this study, which was completed in May of 2002, no nationwide estimates of temporary losses of highway capacity by type of capacity-reducing event had been made.

This report describes the second phase of the TLC study (TLC2). TLC2 improves upon the first study by expanding the scope to include delays from rain, toll collection facilities, railroad crossings, and commercial truck PUD activities in urban areas. It includes estimates of work zone capacity loss and delay for all freeways and principal arterials, rather than for Interstates only. It also includes improved estimates of delays from fog, snow, and ice, which are based on data not available during the initial phase of the study.

Finally, it should be noted that, in the process of expanding the scope and improving the data sources and methodologies for TLC2, it was discovered that a computer-processing error generated incorrect capacity reduction and delay estimates for crashes and breakdowns presented in the original study report (Chin et al. 2002). This error affects all crash and breakdown estimates in that report. The authors have corrected this error and re-calculated the crash and breakdown estimates. These revised estimates are included in the TLC2 report. The specific estimates in the initial TLC report that should be disregarded are given in Table 1 below.

1.2  Objective and Scope

The objective of the TLC study was to develop and implement methods for producing national-level estimates of the loss of capacity on the nation's highway facilities due to temporary phenomena as well as estimates of the impacts of such losses. The objective of TLC2 was to improve these estimates by expanding the scope to include phenomena not included in the first study and to refine the initial estimates in cases where improved data and/or methodologies could be employed.

The scope of TLC2 includes all urban and rural freeways and principal arterials in the nation's highway system for 1999—delays on minor arterials, collectors, and local roads are not included. These roadways accounted for about 54 percent of the highway vehicle-miles of travel (VMT) in 1999 (Fig. 1). Specifically, this study attempts to quantify the extent of temporary capacity losses due to crashes, breakdowns, work zones, weather, sub-optimal signal timing, railroad crossings, toll facilities, and commercial truck PUD activities in urban areas. These events can cause impacts such as capacity reduction, delays, trip rescheduling, rerouting, reduced mobility, and reduced reliability. This study focuses on the reduction of capacity and resulting delays caused by the temporary events mentioned above.  Impacts other than capacity losses and delay, such as re-routing, re-scheduling, reduced mobility, and reduced reliability, are not covered in this phase of research.

It should also be noted that the study does not attempt to estimate capacity losses and delays due to events that occur simultaneously, such as a crash that takes place during a snowstorm or a breakdown that takes place in a work zone. Such coinciding events can often cause more capacity loss and delay than they might have caused if they had occurred separately. The interaction of capacity-reducing events is an area of interest and may be addressed in future research. However, due to time and funding constraints of the initial phases of the study, this interaction was not modeled.

Figure 1. The highways within the scope of TLC2 accounted for about 54 percent of the VMT in the U.S. in 1999.

1.3  Important Terms Used in This Report

Definitions of urban areas, peak periods, and traffic conditions are made as consistent as possible with both FHWA practice and congestion measures published by the Texas Transportation Institute (TTI).

Urban areas are identified in Table HM-72 of FHWA's Highway Statistics 1999 and are classified into the size categories in Table 2.  These FHWA-recognized urban areas are urbanized areas defined by the U.S. Bureau of the Census with populations of at least 50,000—boundaries may be adjusted from the Census definition by local transportation officials.  TTI used 68 of these areas, including the largest urban areas and a sponsor-selected set of smaller areas, in its 2001 Urban Mobility Report (Schrank and Lomax, 2001).  TTI used all FHWA-recognized urban areas in unpublished tabulations for FHWA cited in chapter 11 of this report.

Peak periods are assumed to be from 6:00 am to 9:30 am in the morning and 3:30 pm to 7:00 pm in the afternoon from Monday to Friday, to be consistent with TTI’s annual mobility report. All other times of the day and days of the week are considered off-peak periods.

Traffic conditions for each roadway segment during peak periods are determined by the Volume/Service Flow Ratio (V/SF) information provided in the sample section of the Highway Performance Monitoring System (HPMS) data.  A roadway section is considered congested during the peak periods if its V/SF is greater than 95%. This V/SF is a computed value reflecting peak hour congestion for a sample section. It is used in investment requirements modeling to estimate needed capacity improvements, in the national highway database, and for congestion, delay, and other data analyses. This value is generated by the HPMS software from HPMS data and procedures are described in Appendix N, Highway Performance Monitoring System Field Manual (U.S. DOT/FHWA 2000).

1.4 Organiation of the Report

This report begins by describing the general approach and methodology used in TLC and TLC2 to estimate capacity reduction and delay. It explains the basic concepts of capacity, capacity reduction, and delay and provides a number of derivations, data sources, and assumptions used throughout the study.

Chapters 3 through 10 describe the specific methodology, data, and assumptions used to estimate capacity loss and delay for each event type. These include

• Crashes (both fatal and non-fatal)
• Breakdowns
• Work zones
• Weather events (rain, fog, ice, and snow)
• Sub-optimal signal timing (principal arterials only)
• Highway-railroad crossings (principal arterials only)
• Toll facilities
• Commercial truck pickup and delivery (PUD) activities (urban principal arterials only)

These chapters also present the estimated capacity loss and delay for each event type and provide information on the reliability of the estimates. Capacity reduction and delay statistics are organized by area type (rural, very large urban, large urban, medium urban, and small urban) and time period category (peak and off-peak), where possible. Peak period estimates are further divided into those occurring under congested conditions and those occurring during under un-congested conditions.

Chapter 11 briefly summarizes the estimates for all event types in the TLC2 study and compares them with delay estimates generated by the Texas Transportation Institute (TTI) in their 1999 Urban Mobility Study (Schrank and Lomax 2001), as well as an expanded study performed for FHWA.

The report concludes by identifying areas for further research in chapter 12.

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