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From page 90... ... 78 CHAPTER 5. BENEFIT-COST ANALYSIS METHODS INTRODUCTION The scope of the project was to develop guidelines for the design and treatment of roadside ditches that "consider, at a minimum, risk factors, cost-effectiveness, feasibility, road geometry, and traffic." Here, risk factors are interpreted as those variables that are associated with an increased probability (or risk) Read the entire page →
From page 91... ... 79 resulting guidelines, should be viewed with this scope in mind. The designer's discretion is essential when applying the developed guidelines to local projects. Read the entire page →
From page 92... ... 80 Figure 5.1 provides the basic framework of the BCA method at a glance. It is presented as a schematic flow chart containing an outline of the component models, variables, and data involved in the method and their relationships. Read the entire page →
From page 93... ... 81 Roadway Characteristics • Roadway Type: 2-lane undivided and 4-lane divided highways • Speed Limit and AADT: 55 and 65 mph; low to high volumes • Horizontal and Vertical Alignment: 0, 4.5 & 6 deg; 0, 4 & 6% Roadside Characteristics • Shoulder: 3 width and surface material combinations • Ditch Geometry: see simulation matrix in Chapter 7 • Beyond the Ditch: flat open field with a 6% upgrade Roadside Encroachment Rates • Encro Rates (in number of encro per mile per year) by Roadway Type, Speed Limit, Horizontal Curvature, Vertical Grade and AADT Vehicle Type • 4 Types: Passenger Car, Midsize Sedan, SUV, and Pickup • Distribution: based on crash data Vehicle Encroachment Characteristics at POD • Encroachment Speed Distribution • Encroachment Angle Distribution Driver Control Input • Vehicle Tracking Status at Point of Departure (POD) Read the entire page →
From page 94... ... 82 Roadway and Roadside Specification The BCA method starts with a selection of highways to study based on highway types and PSLs. The goal is to select a small number of combinations of highway types and PSLs that cover a significant percentage of the severe roadside ditch-initiated crashes. Read the entire page →
From page 95... ... 83 Table 5.1. Base encroachment rates for one side of the highway by highway type and projected bidirectional AADT. Read the entire page →
From page 96... ... 84 Figure 5.2. Encroachment rate adjustment factors for curved sections (50) Read the entire page →
From page 97... ... 85 Encroachment Characteristics Model For a specific roadside condition, the probability of an encroached vehicle involved in a crash of a certain severity level depends on a number of encroachment characteristics. Based on available encroachment and crash data, the encroachment characteristics component model establishes real-world encroachment conditions at the POD and the driver's vehicle maneuvering behaviors during the traversal. Read the entire page →
From page 98... ... 86 of the highways where crashes occurred. These travel speeds were estimated by the investigating officer (30) Read the entire page →
From page 99... ... 87 Table 5.6. Encroachment angle probability distribution. Read the entire page →
From page 100... ... 88 characteristics, CarSim was programmed to output key vehicle performance measures during the ditch traversal, which were subsequently used to determine crash severity. These performance measures included extent of lateral and longitudinal encroachments, an indicator variable indicating whether a full recovery of the vehicle is attained, vehicle stability (e.g., rollover or non-rollover) Read the entire page →
From page 101... ... 89 Table 5.8. Relationship Between Severity Index and Severity Distribution Severity Index (SI) Read the entire page →
From page 102... ... 90 MODEL PARAMETER CALIBRATION PROCEDURE A procedure was developed to perform a calibration of some key parameters used in the encroachment characteristics, severity, and cost component models to make sure that the probability-weighted vehicle performance outcomes produced from the developed encroachment model, such as rollover probability, crash severity distribution, and crash cost, were generally consistent with the statistics generated from the crash records. A schematic flow chart of the calibration procedure is presented in Figure 5.4. Read the entire page →
From page 103... ... 91 which the vehicle was able to return to the road or stop safely on the roadside. The consequence of having a high percentage of non-crash encroachments is that a site with a higher ROR crash rate does not automatically indicate a higher ER. Read the entire page →
From page 104... ... 92 Figure 5.4. Model parameter calibration procedure. Read the entire page →
From page 105... ... 93 Table 5.9. Set of simulated ditch configurations selected to represent frequently used configurations in the field. Read the entire page →
From page 106... ... 94 On a per crash basis, the final selected calibration had an average cost of $182,000 per crash, which was somewhat higher than the $127,000 obtained from the crash statistics in Chapter 4. Note that the researchers were unable to reduce the cost further through the adjustments in the calibration without significantly lowering the average rollover probability discussed above. Read the entire page →

From page 90...

... 78 CHAPTER 5. BENEFIT-COST ANALYSIS METHODS INTRODUCTION The scope of the project was to develop guidelines for the design and treatment of roadside ditches that "consider, at a minimum, risk factors, cost-effectiveness, feasibility, road geometry, and traffic." Here, risk factors are interpreted as those variables that are associated with an increased probability (or risk)