U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
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The Colorado Department of Transportation (CDOT) uses two methods for identifying locations with potential for safety improvement: Level of Service of Safety (LOSS) and Diagnostic Analysis. LOSS is based on the concept of Safety Performance Functions (SPF), while Diagnostic Analysis is developed around the idea of statistical pattern recognition.
Design engineers at CDOT pioneered development of the LOSS concept to quantify the magnitude of the safety problem. A crash rate implies a linear relationship between safety and exposure. While crash rates are commonly used to measure safety, they are often misleading since rates change with Annual Average Daily Traffic (AADT). To capture how this rate change takes place, CDOT engineers began calibrating SPFs in the late 1990s based on the work of Dr. Ezra Hauer. By 2001, CDOT had calibrated SPFs for all public roadways in Colorado, which were stratified by the number of lanes, terrain, environment, and functional classification. In 2009, in collaboration with consultants, CDOT developed SPFs for all intersection types.
Development of SPFs supports the conceptual formulation of the LOSS concept. It uses qualitative measures to characterize the safety of a roadway segment in reference to its expected performance. If the number of crashes predicted by the SPF represents normal or expected crash frequency at a specific level of AADT, then the degree of deviation from the norm can be stratified to represent specific levels of safety. To describe road safety from the frequency and severity standpoint, two different SPFs were calibrated: one for the total number of crashes and the other for injury and fatal crashes. When the magnitude of the safety problem is assessed, it is described from the frequency and severity standpoint. The figure (Kononov and Allery, 2003) illustrates the LOSS concept using an SPF calibrated for total crashes expected on the 6-lane urban freeways. The delineated boundary line is located 1.5 standard deviations from the mean, reflecting a Negative Binomial error structure. Four LOSS categories were introduced:
LOSS reflects how the roadway segment is performing in regard to its expected crash frequency and severity at a specific level of AADT. However, it only describes the magnitude of the safety problem; it does not provide any information related to the nature of the problem itself. To investigate the nature of the problem, Colorado uses Direct Diagnostics and Pattern Recognition techniques.
A comprehensive methodology was developed to conduct diagnostic analyses of safety problems. The Direct Diagnostics and Pattern Recognition methods calculate a cumulative binomial probability of the crash types and related characteristics to identify overrepresented elements in the crash data (e.g., dark conditions, overturning vehicles) that may be related to abnormal crash patterns and crash causation. Direct Diagnostics is used for intersection analysis, and Pattern Recognition is used for roadway segments.
CDOT initially used the combination of LOSS and Direct Diagnostics and Pattern Recognition to identify sites with potential for safety improvement only on safety motivated projects. Beginning in 2001, they are applied to all projects at CDOT, including resurfacing, reconstruction, realignment, widening, Environmental Assessments (EA) and Environmental Impact Statements (EIS). CDOT conducts a statewide analysis using Direct Diagnostics and Pattern Recognition and recalibrates SPFs about every five years.
CDOT developed sophisticated predictive and diagnostic tools to maximize potential crash reduction in the state within constraints of available budgets and institutionalized use of these tools throughout the state of Colorado. Over the seven years of application of the advanced methods on all infrastructure and behavioral projects at CDOT, the state has achieved an unprecedented fatal crash reduction of 36 percent, without reduction in travel or increase in safety expenditures. Additionally, these efforts provided substantive analytical and conceptual input for development of the Highway Safety Manual.
Bryan Allery
Colorado Department of Transportation
303-757-9967
bryan.allery@dot.state.co.us
Jake Kononov
Colorado Department of Transportation
303-757-9973
jake.kononov@dot.state.co.us
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