KLK565: Calibrating the Highway Safety Manual Crash Prediction Models for Idaho's Highways: Full Description

ITD Research Project 225; Task Order Number UI-13-01

Principal Investigators:

• Ahmed Abdel-Rahim
• ITD Project Manager
• Brent Jennings

FHWA Project Advisor:

• Lance Johnson

Research Problem Statement:

AASHTO’s Highway Safety Manual (HSM) provides methods to integrate quantitative estimates of crash frequency and severity into planning, project alternatives analysis, and program development and evaluation, allowing safety to become a meaningful project performance measure. The HSM supports ITD progress toward federal and state safety goals to reduce fatalities and serious injuries. As ITD works toward its safety goals, the quantitative methods in the HSM can be used to evaluate which programs and project improvements are achieving desired results; as a result, agencies can reallocate funds toward those that are having the greatest benefit.

In the HSM predictive methods, the total expected crash frequencies for a facility are estimated by combining safety performance functions (SPFs) and crash modification factors (CMFS). The SPFs are first used to calculate estimated crash frequency for a base condition. Next, the estimates are modified by applying CMFs to address non-base condition characteristics for specific segment and intersection locations. The predictive method can be used to estimate safety separately for intersections and segments. The SPF’s included in the HSM, however, were developed using crash data from several states other than Idaho. Because there are differences in driver population, highway geometric characteristics, crash reporting procedures, animal populations, weather conditions, etc., ITD needs to use calibrated SPFs when applying the HSM procedures in Idaho.

Project Objectives:

The current HSM includes crash prediction models for three highway facility types: rural two-lane two-way roads, rural multilane roads, and urban and suburban arterial roads. In this project, we propose to calibrate the SPFs for all three facility types based on their safety performance in Idaho using the state’s crash data. Methods of site selection, the collection of crash and site-specific data, and analysis methods for calibration will be developed and used to develop Idaho-specific HSM calibration factors. An evaluation of the crash severity distribution methods will also be conducted as part of this project. ITD staff can use the calibrated HSM predictive methods to more accurately assess the expected safety performance of different highway segments in Idaho and to assess the safety impact of different design alternatives.

Project Communication Schedule:

Monthly project progress report (ITD 771).

Quarterly project meeting with the project's technical oversight committee.

Needs and Requirements:

This research project aligns with Idaho’s Strategic Highway Safety Plan and supports the top performance measure of eliminating fatalities and serious injuries on all Idaho roadways. 

Required Outputs:

Project final report (task 6 in the project's task list)

Implementation:

The calibrated HSM predictive methods will be used by ITD staff to more accurately assess the expected safety performance of different highway segments in Idaho and to assess the safety impact of different design alternatives.

Expected Outcomes/Savings to ITD:

Benefits of this project include 1) HSM calibration factors using Idaho specific data that lead to more accurate crash prediction models and 2) guidelines for ITD staff on the use of HSM using Idaho specific data.

Monthly Progress Reports (ITD 771) and Task Progress Charts

Draft Report Due: 9/24/14

Project Status: Active

Final Report Due Date: 12/31/14

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