The National Academies Press

Currently Skimming:

Pages 154-180

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 154... ... 154 7.1 Introduction A revised design process that is focused on problem solving using objective performancebased models and methods, and which recognizes inherent differences in project types and contexts, offers considerable benefits. The value and benefits of a performance-based process would hence appear to be as follows: • There would be greater assurance that investments in infrastructure solutions requiring geometric design would produce actual performance enhancements commensurate with the implementation costs, such costs reflecting differences in the project context. Read the entire page →
From page 155... ... Value and Benefits of Improved process 155 positive element of the process and should be the focus of the agency rather than the exercise of explaining why something was NOT done. • The process engenders trust within the community that the owner has engaged them, has been transparent, thorough, and thus developed the best value solution. Read the entire page →
From page 156... ... 156 a performance-Based highway Geometric Design process travel lanes at the intersection approaches to allow for the provision of left-turn lanes. Therefore, the intersection design is the same for both existing and future conditions. Read the entire page →
From page 157... ... Value and Benefits of Improved process 157 The LOS reported in Table 26 is the facility LOS. Note that the NB approach at the south intersection and SB approach at the north intersection experience spillback due to volume exceeding capacity. Read the entire page →
From page 158... ... 158 a performance-Based highway Geometric Design process Conclusions The results of Case Study 1 provide an incomplete picture of the traffic operational and safety effects of the proposed project: • HCM traffic operational analysis procedures indicate that narrowing of the through travel lanes from 12 to 11 feet has no substantive effect on the LOS for motor vehicles in the corridor. • HCM analysis procedures cannot quantify the traffic operational effects of removing the center two-way left-turn lane or providing the bicycle lanes. Read the entire page →
From page 159... ... Value and Benefits of Improved process 159 southbound approach to the southern ramp terminal. The existing eastbound exit ramp and eastbound entrance ramp will be realigned to accommodate the new loop ramp. Read the entire page →
From page 160... ... 160 a performance-Based highway Geometric Design process the signal. The existing three southbound through lanes between the northern and southern ramp terminal remain in the proposed design, but the rightmost lane will be an exit only onto the new loop ramp, while the other two through lanes will proceed through the southern ramp terminal. Read the entire page →
From page 161... ... Value and Benefits of Improved process 161 Figure 54. Design year peak-hour turning movement counts for planned interchange reconfiguration. Read the entire page →
From page 162... ... 162 a performance-Based highway Geometric Design process The LOS assessment was based solely on volume-capacity ratios and control delays. It was assumed that the queue ratios would not cause any movements to operate at LOS F Read the entire page →
From page 163... ... Value and Benefits of Improved process 163 • The ramps on the southern half of the interchange: – In the existing condition, the eastbound diamond exit and entrance ramps. – In the proposed condition, the eastbound diamond exit and entrance ramps as well as the eastbound loop entrance ramp. Read the entire page →
From page 164... ... 164 a performance-Based highway Geometric Design process segment along the speed change lane would decrease and offset some of the increase seen in the segment included in this analysis (which included the speed change lane for the loop ramp in the proposed condition) Read the entire page →
From page 165... ... Value and Benefits of Improved process 165 Traffic Operational Analysis Results Table 33 shows a comparison of the traffic operational LOS for these alternatives, based on HCM methods and assumptions. The full reports from the HCS software are attached. Read the entire page →
From page 166... ... 166 a performance-Based highway Geometric Design process Figure 56. Two horizontal curves with existing and proposed alignments. Read the entire page →
From page 167... ... Value and Benefits of Improved process 167 The safety comparison of the two designs reflects the following: • Widening the paved shoulder, including centerline rumble strips, and flattening the curves results is predicted to result in a substantial (62 percent) reduction in crash frequency (from 8.1 to 3.1 crashes per mile per year) Read the entire page →
From page 168... ... 168 a performance-Based highway Geometric Design process substantially higher than what the current facility can accommodate, so the proposed bypass, which will rebuild the highway along a path to the west of its current alignment and bring it within the city limits, will be a divided four-lane facility. The existing highway alignment will essentially become a local road to serve the homes and farms of this small community. Read the entire page →
From page 169... ... Value and Benefits of Improved process 169 EE EE EE Figure 57. Map of existing conditions overlaid with drawing of proposed highway realignment. Read the entire page →
From page 170... ... Figure 58. 2009 peak-hour volumes and ADT at intersections along existing highway. Read the entire page →
From page 171... ... Figure 59. Design year 2040 peak-hour turning movement volumes and AADT for intersections along both the proposed highway alignment and the existing alignment. Read the entire page →
From page 172... ... 172 a performance-Based highway Geometric Design process • Access points along existing highway in design year 2040 are the same regardless of whether bypass is built. • Access points along the new highway bypass in design year 2040 will serve existing adjacent properties with no access to other routes only; no access points to serve possible future development were considered. Read the entire page →
From page 173... ... Value and Benefits of Improved process 173 Limitations • Intersection of Old Highway and Independence Boulevard in the proposed condition (PI9 in Figure 57) is an all-way stop. Read the entire page →
From page 174... ... 174 a performance-Based highway Geometric Design process Site Type Predicted Average Crash Frequency (crashes/year) N predicted (TOTAL) Read the entire page →
From page 175... ... Value and Benefits of Improved process 175 Facilities) and 17 (Urban Street Segments) Read the entire page →
From page 176... ... 176 a performance-Based highway Geometric Design process The table indicates that the proposed new highway alignment (bypass) would operate at LOS B or C for both directions of travel during both peak periods in 2040. Read the entire page →
From page 177... ... Value and Benefits of Improved process 177 The design process for high-volume urban freeways should allow for the evaluation of alternative cross-section designs that include lane widths of less than 12 feet. There are quantifiable benefits associated with narrower lanes that include the ability to provide additional lanes within limited space, and the cost to reconstruct what are the most costly facilities in the highway system. Read the entire page →
From page 178... ... 178 a performance-Based highway Geometric Design process The above analysis, while hypothetical, demonstrates both the value of considering more flexible lane and shoulder width design values, as well as the concept of optimizing a design for a given available total width dimension. 7.2.6 Case Study 6 A state DOT is undertaking the development of a long range plan for reconstruction of a suburban freeway system, including its interchanges. Read the entire page →
From page 179... ... Value and Benefits of Improved process 179 A design team has been assigned the challenge of developing a geometric design solution for the CTH interchange with the freeway, and to address the issue of the local road bridge. Traffic demand studies and agency policies regarding traffic operations have brought the following conclusions: 1. Read the entire page →
From page 180... ... 180 a performance-Based highway Geometric Design process External stakeholders include existing business owners to the north, developers, and homeowners who live south of the freeway. An important concern is the potential for detours or traffic shifts during construction. Read the entire page →

From page 154...

... 154 7.1 Introduction A revised design process that is focused on problem solving using objective performancebased models and methods, and which recognizes inherent differences in project types and contexts, offers considerable benefits. The value and benefits of a performance-based process would hence appear to be as follows: • There would be greater assurance that investments in infrastructure solutions requiring geometric design would produce actual performance enhancements commensurate with the implementation costs, such costs reflecting differences in the project context.