The National Academies Press

Currently Skimming:

Pages 80-106

The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.

From page 80... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 68 APPENDIX D RELATIONSHIP AMONG URBAN/SUBURBAN ROADWAY CHARACTERISTICS, OPERATING SPEED, AND CRASHES IN AUSTIN, TEXAS OVERVIEW The goal of NCHRP Project 17-76 was to identify the relationships among roadway characteristics (including posted speed limit) Read the entire page →
From page 81... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 69  Gap (seconds) Read the entire page →
From page 82... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 70 Figure 9. Austin road tube traffic counter locations. Read the entire page →
From page 83... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 71 Table 20. Number of segments by functional classification. Read the entire page →
From page 84... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 72 Roadway Geometry and Traffic Control Device Characteristics Data Table 23 lists descriptions of the specific geometric variables considered for this investigation. These variables were primarily chosen based on the findings from the literature review. Read the entire page →
From page 85... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 73 team used Google® Earth to gather the necessary roadway and traffic control device data. The team members gathered the geometric data using the measurement tool, acquired the posted speed limit information using the Street View feature, and used the historical Street View feature to confirm the speed limit that existed at the time the traffic count was made. Read the entire page →
From page 86... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 74 had speed data to allow for the speed and crash analysis. The software used during this effort included ArcGIS and R (212) Read the entire page →
From page 87... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 75 Figure 11. Example of crash assignment using 30-ft buffer along a sample road segment. Read the entire page →
From page 88... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 76 Table 24. Initial speed measures developed. Read the entire page →
From page 89... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 77 Table 25. Descriptive statistics of variables for City of Austin, Texas. Read the entire page →
From page 90... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 78 (a) Average Speed (mph) Read the entire page →
From page 91... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 79 Figure 13 contains several violin plots for average speed. It shows that the average speed is greatest on road segments in rural or park-like areas compared to commercial/retail/industrial or roads in residential areas, which had the lowest mean. Read the entire page →
From page 92... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 80 roads in residential areas; commercial development had the lowest mean. As noted previously, only eight of the 663 segments were classified as rural or park development in this study, which could explain why the kernel distribution is so heavily concentrated around the mean. Read the entire page →
From page 93... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 81 variables including AADT and roadway geometry variables. The research team examined several NB regression models. Read the entire page →
From page 94... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 82 Table 26. Score statistics for model shown in Table 27. Read the entire page →
From page 95... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 83  Presence of a TWLTL was associated with more segment crashes compared to no median, and presence of a raised median was associated with fewer segment crashes compared to no median or TWLTL. Care must be taken in interpreting higher segment crashes at TWLTL segments compared to no median segments because road segments with no median present had lower speeds on average compared to road segments with TWLTL (or raised medians) Read the entire page →
From page 96... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 84 Table 29. Variables with significant effects on KABCO segment (KABCO_NID) Read the entire page →
From page 97... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 85 Table 30. Variables with significant effects on posted speed limit. Read the entire page →
From page 98... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 86 Table 31. Round 2 speed measures considered. Read the entire page →
From page 99... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 87 between crashes and speeds along with roadway characteristics as well as the speed limit simultaneously based on a coherent structural equation modeling (SEM) framework (213) Read the entire page →
From page 100... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 88 results for KABC_NID crashes are presented in Table 33 and Table 34, and the results for KABCO_NID (all) crashes are presented in Table 35 and Table 36. Read the entire page →
From page 101... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 89 and AvgSpd, but with lower Pace and PerOvPSL. This finding implies that PSL has indirect effects on crashes through its effect on those speed measures. Read the entire page →
From page 102... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 90 Table 35. Estimated regression coefficients for KABCO_NID crashes by path analysis (mediator variable: PSL, CoefVar, PerOvPSL, StdSpd, Pace, or SpdAve) Read the entire page →
From page 103... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 91 Table 36. Estimated regression coefficients for KABCO_NID crashes by path analysis (mediator variable: Abs(PSL−Avg) Read the entire page →
From page 104... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 92 roadway characteristic variables in Table 23 was fitted to the dataset consisting of 649 sites, and then the model was refitted after removing variables that were insignificant at α=0.1. The crash rate for the Austin data was calculated and graphed with the speed metric of PSL−Avg (difference between posted speed limit and average speed) Read the entire page →
From page 105... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 93 Results and Discussion The estimated model coefficients for KABC crashes from the Round 3 path analysis are presented in Table 37. The table shows that the mediator variable Abs(PSL−Avg) Read the entire page →
From page 106... ... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 94 Figure 16. Path analysis findings for segments with posted speed limits of 20 to 45 mph. Read the entire page →

From page 80...

... NCHRP Web-Only Document 291: Development of a Posted Speed Limit Setting Procedure and Tool 68 APPENDIX D RELATIONSHIP AMONG URBAN/SUBURBAN ROADWAY CHARACTERISTICS, OPERATING SPEED, AND CRASHES IN AUSTIN, TEXAS OVERVIEW The goal of NCHRP Project 17-76 was to identify the relationships among roadway characteristics (including posted speed limit)