Driver Needs on Two-Lane Rural Highways. Volume 2. Simplified Location of Information Deficiencies (SLIDE) – A Procedure

NHTSA · 1985 · ROSA P / United States. Department of Transportation. Federal Highway Administration. Offices of Research and Development

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Summary

This 1985 Federal Highway Administration report introduces the Simplified Location of Information Deficiencies (SLIDE) procedure, a methodology designed to help state and local highway agencies identify locations on rural two-lane highways where drivers lack necessary guidance information. The research addresses a critical gap in highway safety management: while agencies routinely inventory and inspect the physical condition of traffic control devices, they rarely evaluate whether the existing signage adequately meets driver needs. This deficiency is particularly acute on low-volume rural roads, where accident data is often insufficient to pinpoint hazards, and budget constraints prevent ideal signage coverage. The procedure aims to provide a practical, non-equipment-intensive tool for identifying these "information deficiencies" to prevent accidents caused by driver confusion, erratic maneuvers, or inability to adjust speed and path in time. The SLIDE procedure is grounded in human factors principles, specifically driver expectancy, consistency, uncertainty, and positive guidance. It posits that accidents often result when a driver’s expectations—formed by long-term experience and immediate road conditions—are violated by inconsistent geometry or missing information. The method involves a two-person team driving designated highway sections (5 to 15 miles). The process begins with the driver establishing "initial expectancies" regarding the road’s character, such as alignment, width, and expected signage levels, after driving the first mile. The driver then provides continuous verbal commentary, noting expectations of upcoming features (e.g., curves, intersections), required actions (speed changes, path adjustments), and any uncertainties. Locations where the actual road conditions or lack of signage contradict these expectations are flagged as information deficient. These sites are later evaluated using specific checklists to determine corrective actions. The report details the classification of highways by function and administrative jurisdiction to prioritize surveys, noting that roads with mixed characteristics (e.g., high speed but narrow width) pose the highest risk for expectancy errors. It emphasizes that unfamiliar drivers rely heavily on warning signs, whereas frequent users may ignore them, making consistent signing crucial. The procedure was tested in two regional workshops involving likely users, which confirmed its utility and practicality. The report includes an appendix with checklists for nine specific situations, such as stop-controlled intersections and height/weight restrictions, to standardize the evaluation process. The significance of the SLIDE procedure lies in its shift from passive inventory to active evaluation of driver information needs. By focusing on the cognitive aspects of driving—specifically how drivers process information and form expectations—the method allows agencies to allocate limited resources more effectively. It provides a structured way to identify hazardous locations that traditional accident-based analyses might miss, particularly on low-volume rural roads. The report concludes that implementing such a procedure can enhance safety by ensuring that traffic control devices are placed where they are most needed to support safe speed and path decisions, thereby reducing the potential for accidents caused by information deficits.

Key finding

The SLIDE procedure successfully identifies information deficient locations on two-lane rural highways by systematically evaluating the consistency between driver expectancies and available guidance information.

Methodology

field_study

Provenance

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Topics

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• signage environment
• rail grade crossings
• perceptual countermeasures
• sign visibility legibility