Speed Concepts: Informational Guide

Donnell, Eric T.; Hines, Scott C.; Mahoney, Kevin M.; Porter, Richard J.; Donnell, Eric T. · 2009 · ROSA P / United States. Department of Transportation. Federal Highway Administration. Office of Safety

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Summary

This informational guide, published by the Federal Highway Administration (FHWA) in 2009, addresses the complex relationship between traffic speed, highway design, and safety. The document was motivated by the frequent incompatibility between designated design speeds, operating speeds, and posted speed limits, which often leads to confusion among engineers, planners, and elected officials. The primary objective is to clarify speed-related terminology and explain how these concepts interact to help stakeholders achieve desired operating speeds and set rational speed limits that balance mobility and safety objectives. The guide employs a comprehensive review of existing engineering standards, research conclusions, and technical processes rather than presenting new experimental data. It defines key terms such as designated design speed, inferred design speed, operating speed, and posted speed limits. The authors utilize speed profile plots, tabular summaries, and illustrative examples to document speed-based technical processes. The content draws heavily on the AASHTO *Green Book* for geometric design criteria, including sight distance, vertical curvature, and horizontal curvature with superelevation. It also summarizes the roles of state and local government agencies in planning, design, and enforcement, while highlighting mitigation measures and software tools like the Interactive Highway Safety Design Model (IHSDM). Key findings emphasize that while higher speeds improve mobility by reducing travel times, they significantly increase the risk of injury and fatality. The guide presents clear evidence that crash severity increases with speed, noting that a vehicle’s kinetic energy is proportional to the square of its velocity. Specific data indicates a direct positive correlation between vehicle impact speed and pedestrian fatality probability; for instance, the probability of pedestrian death rises from 5% at 20 mph to 85% at 40 mph. The document clarifies that designated design speed is a tool for determining geometric features, not necessarily the maximum safe speed, and that inferred design speed—derived from actual roadway geometry—can differ from the designated value. It further explains that operating speeds are determined by driver perception of the driving environment, which can be influenced by conflicting signals from roadway geometry and posted limits. The significance of this guide lies in its provision of a unified framework for understanding speed management. By distinguishing between different speed concepts and explaining their technical underpinnings, it enables engineers and officials to better align design features with desired operating speeds. The guide concludes that rational speed limits and effective mitigation measures require a clear understanding of how drivers interpret the driving environment. It serves as a critical resource for resolving inconsistencies in speed management, ultimately aiming to enhance highway safety while maintaining efficient mobility.

Key finding

The probability of pedestrian fatality increases significantly with vehicle impact speed, rising from 5% at 20 mph to 85% at 40 mph.

Methodology

review

Provenance

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Topics

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• speed choice
• speed management
• road geometry
• perceptual countermeasures
• rail grade crossings
• speed distance perception