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NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 15 CHAPTER 4. CONCLUSIONS, OPTIONS, AND SUGGESTED RESEARCH CONCLUSIONS The process of selecting a posted speed limit value for a roadway segment can be influenced by many factors, including engineering concerns, roadway characteristics, human factors such as the way drivers react to the roadway environment in terms of the speed they select, and policies including established agency laws or protocols along with political pressures. The operating speed (engineering) approach is the most common method used in the United States. It relies on the 85th percentile speed with adjustments used to account for existing roadway geometry or crash experience. Many states/local agencies have their own laws/criteria for setting speed limits (many are very detailed). Professionals who perform posted speed limit studies rarely use only the 85th percentile speed (i.e., they use several other factors). Using techniques other than the 85th percentile speed to select the posted speed limit is gaining in popularity in other counties. Several citiesâsuch as Portland, Oregon; Boston, Massachusetts; New York City, New York; Seattle, Washington; and othersâare also experimenting with alternative speed limit setting approaches for city streets. NCHRP Project 17-76 collected insights into how the roadway environment influences operating speed and safety (crashes) through the review of the literature and the collection and analysis of data from two states. Using those insights along with an understanding of different methods being used and currently being considered for the setting of posted speed limits, the research team developed the SLS-Procedure and then automated that procedure with the SLS- Tool and explained that procedure with a user guide (1). The SLS-Procedure uses fact-based decision rules that consider both driver speed choice and safety associated with the roadway. The SLS-Procedure was designed to be applicable to all roadway types and contexts by having a set of unique decision rules for different combinations of roadway types and contexts. The combinations included Limited Access, Undeveloped, Developed, and Full Access facilities. With the SLS-Tool having data entry and results on the same screen along with warning and advisory messages on that same screen, it is a transparent product that should help the user understand what factors influenced the SSL calculations. OPTIONS The overall goal of this research was to develop guidance so engineers can make informed decisions regarding the setting of speed limits. Employing the SLS-Procedure with the user guide (1) and the SLS-Tool can help an agency make those informed decisions and communicate how speed limits are set to a regionâs leadership and citizens. For the SLS-Procedure/SLS-Tool to gain acceptance, it must be introduced to the profession, included in key reference documents or on key websites, and discussed by users. Thus, the research team recommends the following: ï· Identify ways to encourage use of the tool, such as the following: o Identify key groups to receive presentations about the SLS-Procedure/SLS-Tool. o Include the availability of the SLS-Tool and user guide on key websites dealing with speed management. ï· Identify implementation funds that can help to move this research into practice. ï· Make presentations on the research findings and on the availability of the SLS- Procedure, SLS-Tool, and user guide.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 16 ï· Develop and conduct training on the SLS-Procedure, SLS-Tool, and user guide. SUGGESTED RESEARCH Suggestions for additional research include the following: ï· Investigate current statutory speeds in all 50 states and develop a roadmap of how a uniform set of best-practice statutory speeds could be established across the country. ï· Determine needed collaboration among judicial, enforcement, and engineering industries that will result in more uniform setting, enforcement, and adjudication of reasonable speed limits. ï· Identify greater definitions of the other factors used in setting speed limits such that they lead to more uniform application, similar to the use of 85th percentile speed. ï· Conduct outreach to the most experienced state and local personnel who have conducted engineering studies that have set speed limits to refine the process to be efficient and effective. ï· Perform additional research on the relationships among operating speed, roadway characteristics, posted speed limit, and crashes for city streets to confirm the findings in Austin, Texas (see Appendix D), especially with respect to the relationships with 50th or 85th percentile speeds. Speed data are being collected by more groups, which can assist with making this type of research more affordable; however, the speed data need to be accompanied by the number of vehicles present when the speed reading was made. Preferably, the speed data would represent a time period of about 1 hour or less. Speed readings that represent a typical day or more remove too much of the variability in the speed behavior. ï· Conduct similar research for roads with other speed ranges, such as freeways (generally 55 mph and greater), rural highways (subdivided into two speed groups of higher speeds and lower speeds), and local/residential streets. ï· Investigate how the presence of a marked bike lane should influence the SLS- Procedure. While it is logical to consider the need to have lower speed limits when a high number of bicyclists are present, and the SLS-Tool considers that condition, additional research is needed on how to consider bicyclist infrastructure and establish related criteria. For example, is the critical element just the presence of the lane, or is it the separation distance between the bike lane and the vehicle lane, or is a minimum bicycle volume also needed? ï· Establish criteria for pedestrian volume and bicyclist volume. Currently, the SLS- Procedure uses qualitative criteria for pedestrian volume (negligible, some, or high) and bicyclist volume (high or not high). Research is needed to identify appropriate and acceptable values for these levels. These values need to be sensitive to the roadway type and context. In addition, these values may vary by region; for example, transit-heavy areas like New York City may have different values for high levels of pedestrian activities compared to other large cities that do not have such an extensive transit network. ï· Gain additional insights into the relationships with posted speed limit. Research could use the SLS-Tool to set speed limits within a region and then track changes in operating speed and crashes on those streets where the speed limit was modified, along with an appropriate set of reference sites. The study should also explore sites with posted speed limits set by the âsafe systemâ approach.
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 17 ï· Explore alternative approaches for city streets. Several citiesâsuch as Portland, Oregon; Boston, Massachusetts; New York City, New York; Seattle, Washington; and othersâare experimenting with alternative approaches for city streets. NACTO is in the process of developing a speed limit setting process for lower speed streets. The results of the SLS-Procedure/SLS-Tool could be compared to these new methods being developed. Depending upon the magnitude of the difference between these methods, safety evaluations could be conducted using sites where the different methods were employed. ï· Determine other tools that can be used to manage speed along with the setting of defendable speed limits. These tools could include speed feedback signs or increased enforcement, among others. The effectiveness of these tools should be identified and the results communicated to practitioners. While this effort is ongoing by many researchers, perhaps a central clearinghouse could improve the technology transfer. ï· Determine how to integrate target speeds into the decision process (potentially as part of a uniform, best-practice statutory speed framework). Suggestions for additional research related to improvements/expansion of the SLS-Tool include the following: ï· Explore whether the SLS-Tool would be more user friendly (and therefore used more) as a web-based tool rather than a spreadsheet-based tool. NCHRP required that the 17-76 tool be spreadsheet based; however, feedback from some reviewers on a draft version of the tool indicated that a web-based tool may have advantages. Other reviewers preferred the spreadsheet format. ï· Revise the SLS-Tool to be able to handle multiple speed zone studies within a corridor. ï· Provide more options for sidewalk buffer. A reviewer asked if the sidewalk buffer options should be present, not present, or both. Typically, the user is instructed to use the conditions present for the majority of the segment; however, is there justification to expand the sidewalk buffer options, perhaps to include present, not present, or mixed? ï· Have the SLS-Procedure/SLS-Tool consider if the speed zone is within a transitional area. If yes, on the transition area, is the transition zone transitioning up or down in the direction of travel? ï· Determine whether select variables should have greater weight. Should the user be able to adjust those weights for a given segment? ï· Revise to include a new roadway context. NCHRP Research Report 855: An Expanded Functional Classification System for Highways and Streets served as a key reference informing the seventh edition of the American Association of State Highway and Transportation Officialsâ (AASHTOâs) A Policy on Geometric Design of Highways and Streets (commonly known as the Green Book) (17). The new Green Book introduces a change in guidance for state transportation officials by introducing a broader set of land use context classifications (i.e., rural, rural town, suburban, urban, and urban core). These context classifications provide a mechanism for better targeting design solutions to specific contexts while providing needed flexibility to address planning and design needs. The AASHTO Committee on Design has begun planning for the eighth edition of the Green Book and envisions a major restructuring. This includes organizing design guidance by the context classifications listed in the
NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 18 seventh edition with the addition of a context classification, âIndustrial, Warehouse, or Port Roads.â The SLS-Tool should be updated to incorporate this new context classification of âIndustrial, Warehouse, or Port Roads.â ï· Revise the average crash rates. The crash rates in USLIMITS2 (11) originally reflected data from 2000 to 2004 (depending on the state). A revision to USLIMTS2 (15) updated those numbers to reflect data from 2009 to 2012 (depending on the state). The revised average crash rates were incorporated into the SLS-Tool; however, they should be updated again, or per-state or per-regional rates should be identified as funds are available. ï· Explore whether safety performance functions (SPFs) rather than average crash rates should be integrated into the SLS-Procedure. ï· Conduct focus groups to identify other variables that could be considered within the SLS-Procedure. Investigate those variables for suitability for integration into the SLS- Tool. ï· Consider adding a check with respect to pace similar to what is contained in the Florida Department of Transportation (DOT) manual (18). The Florida DOT process is to post the speed limit at or near the upper limit of the 10-mph pace when the observed 85th percentile speed falls above the upper limit of the 10-mph pace. The manual notes that an observed 85th percentile speed that exceeds the 10-mph pace could result from a small percentage of vehicles exceeding the posted speed limit to a greater degree than the average driver traveling within the 10-mph pace.