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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Page 12
Page 13
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2014. Incorporating Travel Time Reliability into the Highway Capacity Manual. Washington, DC: The National Academies Press. doi: 10.17226/22487.
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Page 13

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12 C h a p t e r 1 This final report documents the activities performed during Phase 1 and Phase 2 of SHRP 2 Project L08, Incorporating Travel Time Reliability into the Highway Capacity Manual. The final report articulates the how and why of key decisions made and key activities undertaken during the project so that the logic and rationale are not lost to future researchers and prac- titioners who aim to build on the work completed in this effort. research problem Statement The Highway Capacity Manual (HCM) historically has been among the most important reference guides used by transpor- tation professionals seeking a systematic basis for evaluating the capacity, level of service, and performance measures for elements of the surface transportation system—particularly highways, but also other modes. The HCM is useful for plan- ning, design, preliminary engineering, and operations analy- sis. The manual provides analytic concepts for characterizing traffic flow, capacity, and quality and level of service. It also provides guidance on analyzing facilities, segments, and points for uninterrupted-flow roadways such as freeways and multi- lane highways, and for interrupted-flow roadway elements such as urban streets, signalized intersections, and two-way stop controlled intersections. The HCM distinguishes between capacity and other perfor- mance measures. Capacity is defined as the hourly flow rate at which persons or vehicles can be reasonably expected to tra- verse a uniform section of road. Other performance measures include density, speed, delay, speed, number of stops, queue length, and volume-to-capacity ratio. Travel time reliability is increasingly recognized as an impor- tant mobility performance measure. The HCM does not include a method to address travel time reliability. Nor does it have mobility performance measures or a method to address reli- ability for specific types of facilities such as freeways, multilane highways and urban corridors, and segments such as freeway weaving areas. The HCM has undergone numerous updates since the first version was published in 1950; the most recent iteration is the HCM2010 (TRB 2010a). Much research has been completed within SHRP 2 that provides analytic procedures for comput- ing travel time reliability on urban freeways. These analytic procedures are not in a form that can be applied directly to perform the types of analysis in the HCM. Moreover, gaps exist in SHRP 2 research regarding arterials and corridors. Nevertheless, the SHRP 2 Reliability research is a strong foun- dation for performing the type of analysis in the HCM to address nonrecurring congestion. In sum, analytic proce- dures are needed to incorporate travel time reliability into the methods used within the HCM. project Objectives The main objective of this project is to determine how data and information on the impacts of differing causes of nonrecurrent congestion (e.g., incidents, weather, work zones, special events) in the context of highway capacity can be incorporated into the performance measure estimation procedures contained in the HCM2010. The methodologies in the HCM2010 for predict- ing delay, speed, queuing, and other performance measures for alternative highway designs are not currently sensitive to traffic management techniques and other operation/design measures for reducing nonrecurrent congestion. By includ- ing the effects of incidents, inclement weather, work zones, and demand variability on congestion and reliability, the methodologies produced by the L08 project are sensitive to many traffic management strategies, especially traffic incident management. A further objective is to develop methodologies to predict travel time reliability on selected types of facilities and within corridors. Specifically, to do the following: • Develop travel time reliability as a performance measure in the HCM2010 for freeway facilities; Introduction

13 • Develop travel time reliability as a performance measure in the HCM2010 for urban street facilities; and • Address freeway and urban streets in a corridor context. These procedures should inform planning, preliminary engi- neering, design, and systems operations and management. project tasks The following tasks were conducted as part of SHRP 2 Project L08. Phase 1 Task 1. Conduct a literature review including but not limited to the HCM2010, SHRP 2 research (especially Projects L03 and L07 and other projects in both the Reliability and Capacity focus areas), state procedures, Federal Highway Administration (FHWA) source materials, and interna- tional input. Task 2. Identify gaps in the availability of methodologies needed to satisfy the project objectives. Task 3. Develop a study methodology to satisfy the project objectives. Include a definition of travel time reliability as a performance measure within the context of the HCM2010. Define and interpret key concepts of the pro- posed analytic and predictive procedures. Address reliabil- ity during different time periods. Ensure that the reliability performance measure is field-measurable. Task 4. Develop a data collection plan and methodology that include data fusion requirements, quality assurance, test- ing, and validation. Task 5. Prepare a Phase 1 report that includes a work plan for Phase 2. Present the results of Phase 1 to the Highway Capacity and Quality of Service Committee at a suitable location (a webinar may be used). The contractor may not proceed with Phase 2 until approval is received from SHRP 2. Phase 2 Task 6. Collect data in accordance with the data collection plan/ methodology. Task 7. Analyze the data by following the methodology described in Task 3. Develop models that can be used to assess travel time reliability as a performance measure in the HCM2010 for (a) freeway facilities, (b) urban street facilities, and (c) freeways and urban streets in a corridor context. Task 8. Test and validate models in a manner consistent with the methodologies established in Tasks 3 and 4. Task 9. Develop computational procedures to document each model and make it easy to apply the reliability assessment models independently. Task 10. Present two webinars during the course of the proj- ect, one before data collection and the other after prelimi- nary model development. Record comments and questions from the webinar audiences and consider whether any are useful to the project. Task 11. Prepare a guide that encompasses chapters evocative of the HCM2010 and addresses freeway facilities, urban street facilities, and freeways and urban streets in a corri- dor context. The text should be suitable for potential inclu- sion in a future update of the HCM.

Next: Chapter 2 - Defining and Measuring Reliability »
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TRB’s second Strategic Highway Research Program (SHRP 2) Report S2-L08-RW-1: Incorporation of Travel Time Reliability into the Highway Capacity Manual presents a summary of the work conducted during the development of two proposed new chapters for the Highway Capacity Manual 2010 (HCM2010). These chapters demonstrated how to apply travel time reliability methods to the analysis of freeways and urban streets.

The two proposed HCM chapters, numbers 36 and 37, introduce the concept of travel time reliability and offer new analytic methods. The prospective Chapter 36 for HCM2010 concerns freeway facilities and urban streets, and the prospective supplemental Chapter 37 elaborates on the methodologies and provides an example calculation. The chapters are proposed; they have not yet been accepted by TRB's Highway Capacity and Quality of Service (HCQS) Committee. The HCQS Committee has responsibility for approving the content of HCM2010.

SHRP 2 Reliability Project L08 has also released the FREEVAL and STREETVAL computational engines. The FREEVAL-RL computational engine employs a scenario generator that feeds the Freeway Highway Capacity Analysis methodology in order to generate a travel time distribution from which reliability metrics can be derived. The STREETVAL-RL computational engine employs a scenario generator that feeds the Urban Streets Highway Capacity Analysis methodology in order to generate a travel time distribution from which reliability metrics can be derived.

Software Disclaimer: This software is offered as is, without warranty or promise of support of any kind either expressed or implied. Under no circumstance will the National Academy of Sciences or the Transportation Research Board (collectively "TRB") be liable for any loss or damage caused by the installation or operation of this product. TRB makes no representation or warranty of any kind, expressed or implied, in fact or in law, including without limitation, the warranty of merchantability or the warranty of fitness for a particular purpose, and shall not in any case be liable for any consequential or special damages.

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