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This chapter provides additional technical information on the 13 controlling criteria, including clarifications on when formal design exceptions are required and the potential impacts to traffic operations or substantive safety that a designer should consider when evaluating design exceptions and mitigation strategies.
Traffic operational effects may include the influence of a change in a design dimension on the facility’s capacity, on speed, or on changes in speed or other operating behavior for either the overall traffic stream or certain critical vehicle types. Substantive safety effects may include expected or predicted changes in the crash frequency, severity, or both, associated with an incremental change in a design dimension. For both traffic operational and substantive safety effects, the information provided in this chapter represents a synthesis of research and technical literature.
With respect to substantive safety effects, effects will be described in two ways. Safety performance functions (SPFs) describe the expected crash frequency for a condition or element as a function of traffic volume and other fundamental values. SPFs are usually expressed as an equation or mathematical function. Accident modification factors (AMFs) describe the expected change in crash frequency (total or particular crash types) associated with an incremental change in a design dimension. AMFs may be shown in tabular form or in some cases as a simple function. They are expressed as a decimal, with an AMF less than 1.0 meaning the crash frequency would be lower and an AMF greater than 1.0 meaning the crash frequency would increase. So, for example, an AMF of 0.95 means a reduction in expected crash frequency of 1.0 – 0.95, or 5 percent.
Designers should be aware that traffic operational and substantive safety effects associated with incremental design dimensions will vary by facility type and context. For example, the change in capacity associated with a 1-foot change in lane width is different for a two-lane rural highway versus urban freeway versus signalized intersection approach. So, considering a design exception in each case will mean a different operational effect should be expected.
Designers should also be mindful of the fundamental concept of exposure. As discussed in Chapter 2, exposure to traffic volume, length of highway, and duration of the design exception are of primary importance. A 5 percent reduction in capacity or expected increase in crash frequency will in many cases be negligible when converted to an annualized value; but in other contexts (say, a high-volume urban freeway) a 5 percent reduction in performance may translate to significant annual impacts.
The information presented in each section is intended to provide the reader with a basic awareness and understanding of expected effects of design exceptions. At the end of the discussion of each criterion, a list of resources is provided for further consultation.