APPENDIX H. SUGGESTED MODIFICATIONS TO STANDARD REFERENCES

This appendix describes suggestions for modifications where the findings from this project may be incorporated into commonly used reference documents relevant to the research.

AASHTO GREEN BOOK

• Section 2.3.6.3 of the 2018 Green Book (AASHTO 2018) focuses on design speed and it includes a brief mention of target speed. Modification of that section is part of a currently ongoing process to revise the Green Book to add new and updated guidance on the geometric design process. Because of that ongoing revision, a discussion of specific modifications to existing sections of the 2018 Green Book would be moot, as many of those sections are being changed already. Instead, the research team suggests that the subsequent edition of the Green Book add a separate section on target speed that incorporates this project’s definition of target speed and adds discussion on incorporating target speed into the design process. Supporting material in this new section on target speed would discuss potential speed management treatments in the context of achieving an attainable target speed.

TRB HIGHWAY CAPACITY MANUAL (HCM)

• The 6th Edition of the HCM (TRB 2016) defines target speed as “the speed at which a specific driver prefers to drive”. This definition in Chapter 7 is provided in the context of a goal of a zero-delay condition, though the HCM acknowledges that different tools have different definitions of target speed. The research team suggests that subsequent editions of the HCM be modified to either use a term other than target speed (e.g., “driver’s desired speed”) or to further qualify the existing term as a “zero-delay speed” to minimize confusion over multiple meanings of the term and improve consistency across the profession.
• The target speed term is repeated in Chapter 11 without a definition or reference to Chapter 7. Chapter 11 focuses on freeway operations, which is an uninterrupted flow context different from other roadway scenarios and contexts. It is suggested that the definition of target speed in Chapter 11 be specified in the text and, if intended to have the same definition as used in Chapter 7, be specified by reference to promote consistency. The suggestion in the previous bullet for either modifying the definition of target speed or changing the term in Chapter 7 also applies to this use in Chapter 11.

FHWA MANUAL ON UNIFORM TRAFFIC CONTROL DEVICES (MUTCD)

• The MUTCD (FHWA 2023) includes warrants on when to use center line and edge line markings, for example, “center line markings shall be placed on all paved undivided two-way urban arterials and collectors that have a traveled way of 20 feet or more in width and an ADT of 6,000 vehicles per day or greater.” The analysis within NCHRP 15-76 demonstrated that speeds are notably slower on segments

• without center line (or edge line) markings for roads with ADTs or travel way width values similar to, and in some cases, greater than, those listed in the MUTCD. When considering road segments with ADTs and travel way widths that are 120 percent of the MUTCD values (i.e., ADTs of 7200 or less and travel way widths of 24 ft or less), the road segments without center line or edge line markings still had statistically significant lower operating speeds. This finding could support modifying the ADT and travel way values in the MUTCD; however, a study on the crash differences for segments with and without center line or edge line markings and these ADT and travel way values should be conducted to determine if there are safety differences due to the absence of the markings.

ITE DESIGNING WALKABLE URBAN THOROUGHFARES: A CONTEXT SENSITIVE APPROACH

• The research team suggests updating the definition of target speed as shown on page 108 of the 2010 ITE Recommended Practice (ITE 2010). The description of target speed in the ITE RP, while accurately discussing target speed relative to the context of the roadway and its adjacent land use, could add more guidance about incorporating target speed into the design process. The content of the RP also reflects the content of other guidance documents in effect at the time the RP was published; as those other documents, such as the Green Book, are revised, the guidance in future versions of the RP relative to those documents can also be revised.

ITE IMPLEMENTING CONTEXT SENSITIVE DESIGN ON MULTIMODAL CORRIDORS: A PRACTITIONER’S HANDBOOK

• The ITE document Implementing Context Sensitive Design on Multimodal Corridors: A Practitioner’s Handbook (ITE 2017), was approved in 2017 as an informational report, for the purpose of complementing the 2010 RP. The description of target speed in this document says that target speed is “the speed at which the community desires motorists to travel”, though the text does not provide guidance on who is considered part of the community that would make that decision or how that decision should be made. The text does provide a reference to the 2010 RP for a detailed comparison of design speed and target speed, and it contains a note that AASHTO recommends target speeds on urban arterial streets of 20 to 45 mph in the 2011 Green Book. The research team suggests that this content be revised in future versions of the handbook or subsequent documents that may supersede it. The suggestion for the revised content would include the definition of target speed described in this project and specify that transportation practitioners make decisions on selecting and designing for target speed as part of the design process. Additional text from this research report and from the 8th edition of the Green Book can be used to provide a more detailed description of target speed in an updated version of the ITE handbook.