Calibrating Safety

FHWA’s 2011 Interactive Highway Safety Design Model features a new calibration utility for crash prediction.

Highway design calls for a careful assessment of safety and operational impacts.

To support decisionmaking in this process, project managers, designers, and traffic and safety reviewers in State and local highway agencies and engineering consulting firms employ the Federal Highway Administration’s (FHWA) Interactive Highway Safety Design Model (IHSDM), a suite of software tools that expedites evaluation of existing highways and proposed design alternatives. 

On September 29, 2011, FHWA released version 7.0.0 of IHSDM, which boasts a new calibration utility for its crash prediction module, a redesigned intersection editor, and improved software features in evaluation modules—a graphical user interface, data, help/documentation, and system administration tools.

Interactive Highway Safety Design Model (IHSDM) calibration utility interface for entering site data.

Crash Prediction Module results for an alternative design for IHSDM Pike.

“How to quantify why one design is safer than another is always a challenge,” says Dr. Clayton Chen, a program manager and highway research engineer in FHWA’s Office of Safety Research and Development. “Now we have a calibration tool that can enable greater confidence in crash prediction estimates.”

The new IHSDM calibration utility addresses the reality that crash frequencies may vary substantially from one jurisdiction to another, due to differences such as climate, driver and animal populations, and crash reporting thresholds and systems. As the American Association of State Highway and Transportation Officials’ (AASHTO) Highway Safety Manual points out, “For predictive models to produce results that are meaningful and accurate for each jurisdiction, it is important that the models be calibrated for application in each jurisdiction.”

For example, a calibration factor of 1.0 means that a State would have the same experience as the States that provided data to develop models for crash prediction. A calibration factor of 1.5 means a State can expect to see 50 percent more crashes than in States where the model is developed for the same type of highway and conditions. A calibration factor that is less than 1.0 means fewer crashes.

According to Mike Dimaiuta, an on-site contractor who manages the Geometric Design Laboratory at FHWA’s Turner-Fairbank Highway Research Center (TFHRC), the calibration utility will assist agencies in implementing the calibration procedures described in the Highway Safety Manual. “Calibrating the crash prediction models allows for enhanced distinctions between design alternatives,” he says.

With greater confidence in the expected crashes predicted by the models, IHSDM output could be used in benefit/cost analyses, where the benefit is the savings realized from the reduction in crashes for one alternative versus another. For example, if Alternative A is expected to experience 50 fewer crashes than Alternative B over a certain period of time, then the benefit of selecting A is obtained by using a breakdown of fatal, injury, and property damage only crashes and assigning costs to each crash severity level.

First released in 2003, IHSDM has six evaluation modules: crash prediction, policy review, design consistency, intersection review, traffic analysis, and driver/vehicle. The modules provide quantitative safety performance measures, check designs against applicable guidelines, and diagnose factors contributing to safety.

IHSDM, which is supported by FHWA’s Geometric Design Laboratory at TFHRC, evaluates two-lane rural highways, multilane rural highways, and urban and suburban arterials. It is an essential resource in support of AASHTO’s Highway Safety Manual, Part C: Predictive Method.