Geometric design refers to the selection of roadway elements such as the horizontal alignment, vertical alignment, cross section, and roadside of a highway or street. A good geometric design must provide appropriate levels of mobility and land use access, while at the same time maintain a high degree of safety. The balance between mobility/accessibility and safety is often reflected by the “allowed” vehicle speed on the roadway element. The design speed is usually used to determine geometric features of a new road during road design. In the 1994 edition of the AASHTO Green Book, the design speed was defined to be “the maximum safe speed that can be maintained over a specified section of highway when conditions are so favorable that the design features of the highway govern.” The assumed design speed should be a logical one with respect to the topography, the adjacent land use, and the functional classification of highway. In 2004, the definition was changed to “a selected speed used to determine the various geometric design features of a roadway.” Hence, design speed is critical for the design of a roadway, especially in choosing super-elevation rates and radii of curves, sight distance, and the lengths of crest and sag vertical curves. Roads with higher travel speeds require sweeping curves, steeper curve banking, longer sight distances, and smoother hill crests and valleys. The operating speed of a road is the speed at which vehicles generally operate on that road. In the same 1994 edition of the AASHTO Green Book, the operating speed was defined as “the highest overall speed at which a driver can travel on a given highway under favorable weather conditions and under prevailing traffic conditions without at any time exceeding the safe speed as determined by the design speed on a section-by-section basis.” In July 2001, however, AASHTO revised their definition in the new edition of the Green Book and defined it as “the speed at which drivers are observed operating their vehicles during free-flow conditions.” The 85th percentile of observed speeds is the most frequently used measure of operating speed. Currently, the choice of design speed value in many states is often based on traffic volume and roadway functional classification, and this may cause a discrepancy with the actual operating speed of that same roadway. For example, a very low-volume road functionally classified as local may have a design speed of 30 mph. However, the regulatory posted speed may be 55 mph, and the operating speed may be even higher. The ranges of such deviation are partially reflected in the design-class flow chart in Figure 1, where the shown speed is generally the anticipated operating speed—often used as design speed. (Note, however, that the actual operating speed is often higher than the anticipated operating speed.) The discrepancy between design speed and operating speed is potentially problematic from a safety point of view. Safety features of geometric design (such as clear zone and guardrail length) are determined based on the lower design speed rather than the actual operating speed. Of particular concern are locations with a sheer drop-off, such as an 8 ft deep box culvert just outside the policy clear zone. The clear zone for local roads can be as low as 6 ft for roads with average daily traffic (ADT) - 400, whereas 55 mph design speed on a 750 ADT road with relatively flat front slopes may require a clear zone of 16 ft. Is it reasonable to accept a clear zone based on design speed, or is a higher design based on operating speed more appropriate- There is a question about whether legal speed or operating speed is more appropriate than design speed when designing clear zones and safety features. This is especially true for low-volume roads with high operating speeds. Are coefficients used in safety design for higher ADT roadways acceptable, or should they be calibrated for lower-volume roads- Should a researched and risk-based consideration of safety features based on design speed vs. operating speed be implemented- At this time, there are locations such as a deep culvert just outside a clear zone (as noted previously) that may not meet policy for a higher design speed. Some engineers find the clear zone policy acceptable for the design speed on low-volume local roads, but others are uncomfortable with the policy when the operating speed is higher, based on engineering judgment. This study aims to quantify the impact of design speed vs. legal/posted speed and operating speed on highway design safety. The results will help the Illinois Department of Transportation (IDOT) better assess the impact of design speed choices, improve its design practice, and ultimately improve safety without compromising mobility and accessibility. More specifically, this study will answer the question of whether legal speed or operating speed is more appropriate than design speed when designing safety features. We will have a better understanding on whether the coefficients used in safety design for higher ADT roadways are acceptable for lower-volume roads. We will also develop a new risk-based analysis framework for safety features based on design speed vs. operating speed. Also, a benefit–cost analysis will be conducted. The rest of the report is organized as follows: Section 2 provides a comprehensive review of the literature that links design speed, geometric design features, operating speed, and safety impacts. Section 3 describes the methodology we propose to quantitatively analyze the safety impacts of speed harmonization. Section 4 shows an illustrative example to demonstrate our framework. Section 5 discusses directions for future research. (Author/publisher)
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