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FHWA Home / Safety / Geometric Design / Publications / Mitigation Strategies For Design Exceptions

Resources to Support Substantive Safety Analysis and Decision


Resources are available and under development that support good decision making by helping designers consider both nominal and substantive safety, evaluate design alternatives (including potential design exceptions), and quantify impacts to safety and traffic operations.

The Interactive Highway Safety Design Model

The Interactive Highway Safety Design Model (IHSDM) is a software tool developed by FHWA that can assist designers with evaluating design alternatives for two-lane rural highways.  The software is used to generate quantitative information on the safety and operational effects of geometric design alternatives.  The current version of IHSDM consists of five evaluation modules:

  1. The Policy Review Module checks design elements for compliance with geometric design criteria (in effect, it produces a ‘nominal safety’ analysis).  For projects on existing roadways, it can provide an initial assessment of how the existing geometric design compares to current design criteria.  The module can be used throughout the design process to check compliance with design criteria.

  2. The Crash Prediction Module estimates the frequency and severity of crashes that can be expected on a highway based upon its geometric design and traffic characteristics.  This module can help identify potential improvement projects on existing roadways, compare the relative safety performance of design alternatives, and assess the safety cost-effectiveness of design decisions.

  3. The Design Consistency Module helps diagnose safety concerns at horizontal curves by providing estimates of the magnitude of potential speed differential.  Design consistency evaluations provide valuable information for diagnosing potential safety issues on existing highways.  These evaluations also provide quality-assurance checks of both preliminary and final alignment designs.

  4. The Intersection Review Module evaluates an existing or proposed intersection geometric design to identify potential safety concerns and suggest possible treatments to mitigate those concerns.

  5. The Traffic Analysis Module estimates traffic quality–of–service measures for an existing or proposed design under current or projected traffic.  This module is particularly useful during project scoping and preliminary engineering to evaluate the operational performance of alternatives to two-lane cross sections, including passing lanes, climbing lanes, and short four-lane sections.

IHSDM software may be downloaded free of charge through the IHSDM public software Web site: http://www.ihsdm.org/.

The Highway Safety Manual

The Highway Safety Manual (HSM) is a resource currently under development to provide a comprehensive manual for highway safety.  The HSM will be a synthesis of validated highway research, as well as practical information and tools to more quantitatively incorporate safety into the decision-making process.  This will include analytical methods for predicting the impact of proposed alternatives on safety.  For more information on the HSM, see http://www.highwaysafetymanual.org/.

A Guide for Achieving Flexibility in Highway Design

This AASHTO Guide provides information on the background, assumptions, and methods for how current design criteria have been developed.  The Guide also provides information on how traffic volume, traffic composition, speed, location, other design elements, and other variables influence the level of risk associated with deviations from design criteria.  A better understanding of these two issues can improve decision making.    

NCHRP Report 500-Series Safety Guides

FHWA and AASHTO are leading a national effort to reduce the nation’s fatality rate to 1.0 per hundred million vehicle miles traveled (HMVMT)—from a current nationwide rate of 1.5 per HMVMT.  This will result in approximately 9,000 fewer fatalities per year. The AASHTO Strategic Highway Safety Plan, developed to guide this national effort in a coordinated, comprehensive manner, lists 22 safety emphasis areas, see the Web site http://safety.transportation.org/plan.aspx.

A series of safety guides to support implementation of the Safety Plan is being developed through the National Cooperative Highway Research Program (NCHRP).  The guides focus on each emphasis area in the Safety Plan by providing technical discussions on potential strategies and programs for reducing highway fatalities and injuries.  A comprehensive approach is presented, with detailed discussion on each emphasis area from the perspective of programs related to the "4 Es":  Engineering, Education, Enforcement, and Emergency Medical Services.  

In terms of design exceptions, the guides can be a useful resource for identifying and evaluating mitigation strategies.  After identifying overrepresented crash types, designers can refer to the appropriate Report 500 volume for potential countermeasures (Figure 2).

The guides are available in printed form from the Transportation Research Board bookstore (http://gulliver.trb.org/bookstore/), and PDF versions are available at no cost from the AASHTO Strategic Highway Safety Plan Web site http://safety.transportation.org/plan.aspx.

FIGURE 2. NCHRP Report 500 Series.


NCHRP Report 500 Series.

Figure 2 shows a typical report cover for the National Cooperative Highway Research Program (NCHRP) as part of its Report 500 series.  This volume is entitled Volume 13, A Guide for Reducing Collisions Involving Heavy Trucks.


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Page last modified on April 1, 2019
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