CHAPTER 4. CONCLUSIONS, RECOMMENDATIONS, AND SUGGESTED RESEARCH

CONCLUSIONS FROM REVIEW OF LITERATURE AND CURRENT PRACTICES

The research team reviewed relevant research literature and current guidance and policy documents that discuss the use of target speed in the design process. Key findings from that review are as follows:

• Speed can be described and measured in many ways. It is important to be clear on what speed measurement is being used, how it is defined, and how it relates to other speed measures.
• The use of target speed is increasing in the profession, though it is often not well defined or is described in the context of another speed measure. When the term is defined, the definition often includes the phrase “the highest speed at which vehicles should operate on a thoroughfare in a specific context” or “the operating speed that the designer intends for drivers to use”. Those definitions emphasize the operational performance of target speed but do not explicitly tie it to the design process. Based on the common uses of the term and its application within existing literature and policy, as well as the need to place the term in the context of the design process, the definition for target speed developed by the research team for this project is as follows: a speed selected for a roadway during the design process that guides planning, design, and operational decisions and results in a roadway with a driving environment that encourages drivers to operate at or below that selected speed. Target speed can be used in the design process in conjunction with more established speed concepts such as design speed, operating speed, and posted speed limit.

CONCLUSIONS FROM INVESTIGATION OF RELATIONSHIPS BETWEEN OPERATING SPEED AND ROAD ELEMENTS

Several combinations of variables were explored during this project. The research team decided to focus on variables within the following five situations:

• Evaluation using only the posted speed limit variable. This evaluation could provide insight into the influence of the number on the speed limit sign.
• Evaluation using variables influenced by transportation professionals. This evaluation included variables that could be under the influence of a transportation professional.
• Evaluation using all available variables including demographics variables. This evaluation included any available variable that could help to identify conditions that are associated with different operating speeds.
• Evaluation focusing on average lane width. This evaluation explored the relationship between lane width and operating speed for segments with average lane width between 8 and 13 ft.
• Evaluation focusing on center line and edge line markings. This evaluation focused on the presence or absence of center line or edge line markings on the subset of the database containing two-lane roads with no median, an average lane width of no more than 13 ft, no mix of curbs and shoulders within the segment, and a PSL of 25 to 35 mph.

In these models, the response variable was ffspd, and state was included as a random effect. Note that the response variable, ffspd, reflects the 67^th percentile speed for the entire year and not the traditional 85^th percentile speed (i.e., daytime only, free flow, etc.). Therefore, the results of these investigations should not be used for speed prediction; rather, the results provide an appreciation of what roadway characteristics may be influencing the speed drivers are selecting on a given roadway segment. Key results from the analyses are as follows:

• In the posted speed limit evaluation, posted speed limit explained about 30 percent of the variability in operating speed for the 6,327 segments included in the database.
• In the transportation professionals evaluation:
  • Faster speeds were associated with higher volumes, wider bicycle lanes, increased building setback, presence of shoulder, increased shoulder width, and higher posted speed limit.
  • Slower speeds were associated with increased building height, presence of curb, higher access density, on-street parking, higher National Walkability Index score, higher roundabout density, higher traffic signal density, presence of speed humps.
  • Compared to segments with no bike lane, speeds were slightly lower when the bike lane was separated with a buffer and no vertical element, speeds were slightly higher when the bike lane was separated with pavement markings only, and speeds were higher when the bike lane was separated with a vertical element.
  • Speeds were found to be slower on 2-lane roads as compared to 4-lane roads by 3.4 mph.
• In the demographics and transportation professionals evaluations:
  • Faster speeds were associated with more occupied housing units and a larger percent of two-plus-car households.
  • Slower speeds were associated with increased building height and higher National Walkability Index score.
• In the evaluation focusing on average lane width, on 4-lane road segments with average lane widths between 8 and 13 ft, the variable was not found to be statistically significant. Average lane width was significant on 2-lane undivided segments for segments that had between 8 and 13 ft average lane widths.
• In the evaluation focusing on center line and edge line markings, for two-lane roads with posted speed limit between 25 and 35 mph, the absence of center line markings was associated with slower operating speeds, as was the absence of edge line markings.

Overall, the analyses in this study found these variables to have the greatest influence on operating speed on roadway segments with a 25 to 40 mph posted speed limit:

• Signal density. This variable ranged between 0 and 22 signals per mile with an associated difference of 15.1 mph between when 0 signals per mile were present compared to when 22 equivalent signals per mile were present.
• National Walkability Index. For this database, the range of values was 2.83 to 20.0. The segments with a value of 20 would have operating speeds that are about 8.2 mph lower as compared to those segments with a NatWalkInd of 2.83. The influence of intersection density, proximity to transit stop, and diversity of land use are considered within the National Walkability Index.

• Access density. More access points were associated with slower speeds, as anticipated. For the range of access points present in this database (0 to 116 driveways or unsignalized intersections per mile), the speed difference was 4.1 mph slower for the segments with 116 access points per mile than for the segments with no access points.
• Building height. The segments with the tallest buildings were estimated to have speeds that were 5.2 mph slower.
• Building setback. The segments with buildings within a few feet had operating speeds that were estimated to be 2.4 mph slower.
• Presence of speed humps. Speeds were slower when a speed hump is present on the segment by about 3.6 mph. This database included 88 segments with speed humps that were on 25-mph (40 segments), 30-mph (34 segments), and 35-mph (14 segments) roads. None of the 40-mph segments had a speed hump.
• Shoulder width. Speeds were higher with increasing shoulder width. For the range of shoulder width values present in this database (0 to 18 ft), the speed change was 5.6 mph between the two extents of shoulder width.
• AADT. For the range of AADT values present in this database (350 to 59000 veh/day), speed is 3.2 mph faster on the roads with the largest AADT. The finding that speeds are higher on roads with larger AADT is counterintuitive; therefore, these results may suggest that AADT is a surrogate for other factors, rather than an indication that higher traffic volumes produce higher speeds.
• Center line markings and edge line markings. When considering the operating speed on two-lane undivided road segments with a 25 to 35 mph posted speed limit and average lane widths between 8 and 13 ft, the analysis found that the presence of edge line and center line markings was associated with higher operating speeds on the order of 2.1 mph for edge line and 5.2 mph for center line markings.
• Average lane width. The average lane width variable was found to be not statistically significant for segments with 2 lanes per direction. Average lane width was found to be significant for two-lane undivided roads (roads with 1 lane per direction). The speed difference between an 8-ft and 13-ft lane on two-lane undivided streets without bike or parking lanes is about 1.3 mph. These road segments could have curbs or shoulders with posted speed limits of 25 to 35 mph.

Variables that may have potential to be influential, but were not statistically significant or the influence was of a smaller difference in operating speed than anticipated for this database, included the following:

• Roundabout Density.
• Posted Speed Limit.
• Bicycle Lane Width.
• Number of schools within 0.5 mile.

The full details of the analyses, findings, and conclusions are found in Appendix F and Appendix G.

CONCLUSIONS FROM PRACTITIONER’S GUIDE

The research team developed a practitioner’s guide for incorporating target speed into the design process. The guide contains a brief introduction, a chapter on how to determine target speed, discussion of factors that affect speed, and a selection of case study examples. Key conclusions from the practitioner’s guide are as follows:

• It is important to select a target speed early in the design process so that it can guide decisions on design elements.
• When selecting a target speed, the target speed value should be realistic and attainable for the context and existing conditions.
• The road agency should conduct a review of the segment to determine whether the treatments were effective in achieving the target speed and consider whether additional treatments may be appropriate.

RECOMMENDATIONS

Based on the activities and findings from the research, the research team makes the following recommendations:

• Road agencies (e.g., state DOTs, city street departments, etc.) should include target speed as a component of their process for designing roadways in their jurisdiction. This should include the selection of an attainable target speed value, consideration of applicable treatments or road elements that promote achieving that target speed, and a review of the performance of treated roadways to determine effectiveness of treatments and consider if additional treatments would be beneficial.
• Design policy and guidance documents should include a definition of target speed that acknowledges target speed in the context of the design process. The definition of target speed in this project provides a useful example.
• Practitioners involved in the design process (e.g., roadway designers, planners, traffic engineers) should each consider target speed in their respective phases of the design process to promote a common outcome that produces a roadway consistent with the selected target speed. This includes consideration of land use and access density in the planning phase, cross-section and geometric features in the design phase, and appropriate traffic control devices in the operations phase. The conclusions that describe relationships between operating speed and road elements provide examples of items that practitioners should consider in each phase of the design process.

SUGGESTED RESEARCH

The results and findings describe analyses of multiple models for a database of 43 variables from 6,327 roadway segments in 8 states. Even with such a comprehensive dataset, some limitations exist, which provide opportunities for future research, as noted below.

• A research project could identify the types of countermeasures currently being used to manage speeds on 30 to 40 mph roads (or other speed ranges if lower or higher speeds are of interest). The research would document the current knowledge with regards to the effectiveness of these countermeasures.
• The purpose of this NCHRP project, as well as the nature of the data and the analyses, directed the research to produce results that describe factors associated with

• differences in operating speed. For example, signal density ranged between 0 and 22 signals per mile with an associated difference of 15.1 mph between when 0 signals per mile were present compared to when 22 equivalent signals per mile were present. This research did not produce speed prediction, and the results as described above are not intended for speed prediction. A future project could focus on developing speed prediction equations. Note, because of the large number of influential elements, unique studies may be needed to develop speed prediction equations for the broad range of roadway types (e.g., urban core, suburban, etc.), number of lanes (e.g., 2, 4, 6, etc.), type of traffic control devices in the area (e.g., signal, uncontrolled), transit activity (e.g., none, primary bus route), access density, traffic operations (e.g., one-way or two-way), bike facilities, and other conditions.
• Another approach to understanding how roadway elements are related to speed is to have future projects that investigate the results of implementing one or more speed management treatments, comparing conditions before and after implementation, and generating speed adjustment factors (also called speed modification factors). Such a project could be useful when an agency is trying to select a specific countermeasure for decreasing speed on an existing street. Research of this nature would also be consistent with the type of review and study that an agency would conduct after implementing a target speed program on one or more segments to affect operating speed, to determine the effects and effectiveness of the treatments used.
• The analyses did not include a component for identifying interaction effects. With 43 variables in the analysis, there is an opportunity for multiple variables to have an effect in combination that is different from their individual effects. Future research could study the nature and magnitude of these interaction effects.
• The current approach provided insights into the relationship with speed for several static site characteristics. Site characteristics that could be considered dynamic (e.g., pedestrian and bicyclist activity or frequency of buses) can also have a significant influence on speed. Future research efforts could examine how these activities influence operating speed.
• The MUTCD includes warrants on when to use center line and edge line markings. 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 ways with values less than those listed in the MUTCD. When road segments with ADTs and travel way widths that are 120 percent of those 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.
• A general assumption in the profession is that decreasing lane width should result in slower speeds. That assumption was investigated in this study with nuanced results. With the full database that considered a typical cross-section of road segments with 25- to 40- mph posted speed limits, the average lane width variable either produced counterintuitive results (higher speeds with smaller widths) or was not significant. Contributing to that finding, perhaps, was that several segments had average lane

• width values greater than 13 ft, which would typically be considered wider than normal for a street. A review of a sample of those sites showed that unmarked parking was included in the average lane width calculation. The evaluations that focused on a subset of the segments that had average lane widths of 13 ft or less found that average lane width was significant for 2-lane undivided roads but not for 4-lane roads. A theory is that the presence of the wider driving surface results in faster speed, even though the average lane width is the same for the 2-lane and 4-lane examples. These findings demonstrate that the influence of average lane width on driving speed will change depending upon other conditions. A future research study could focus on understanding what those conditions are and how influential each is.
• This project focused on the design process and how road elements affect vehicle speeds. As such, the scope of the project did not include consideration of crashes or other measures of safety, though the effects of speed on safety have been the topic of much research and discussion within the profession. A future research project could focus on the reduction of crashes or other safety effects of target speed treatments to provide a well-rounded body of knowledge on the use of target speed in the design process.
• A future research project could investigate the relationship between operating speed and road elements on streets with one-way operations.
• Develop case studies to illustrate experiences with implementing countermeasures for managing speeds on urban streets.