The study area was the state of Utah. Crash data from 2005-2007 on 157 selected study segments provided a 3-year observed crash frequency to obtain a calibration factor for the HSM SPF and develop new SPFs. The calibration factor for the HSM SPF for rural two-lane two-way roads in Utah is 1.16. This indicates that the HSM underpredicts the number of crashes on rural two-lane two-way roads in Utah by sixteen percent.

The study area was the state of Utah. Crash data from 2005-2007 on 157 selected study segments provided a 3-year observed crash frequency to obtain a calibration factor for the HSM SPF and develop new SPFs. The calibration factor for the HSM SPF for rural two-lane two-way roads in Utah is 1.16. This indicates that the HSM underpredicts the number of crashes on rural two-lane two-way roads in Utah by sixteen percent.

This report documents the calibration of the Highway Safety Manual (HSM) safety performance function (SPF) for rural two-lane two-way roadway segments in Utah and the development of new models using negative binomial and hierarchical Bayesian modeling techniques. Crash data from 2005-2007 on 157 selected study segments in Utah provided a 3-year observed crash frequency to obtain a calibration factor for the HSM SPF and develop new models. The calibration factor for the HSM SPF for rural two-lane two-way roads in Utah is 1.16, indicating that the HSM underpredicts the number of crashes on these roads by 16 percent. Negative binomial regression was used to develop four new models, and one additional model was developed using hierarchical (or full) Bayesian techniques. The empirical Bayes (EB) method can be applied with each negative binomial model because the models include an overdispersion parameter used with the EB method. The hierarchical Bayesian technique accounts for high levels of uncertainty. Because the hierarchical Bayesian model produces a density function of a predicted crash frequency, a comparison of this density function with an observed crash frequency can help identify segments with significant safety concerns. Each model has its own strengths and weaknesses, which include its data requirements and predicting capability. This report recommends that UDOT use the negative binomial model with transformed average annual daily traffic (AADT) at a 95 percent confidence level (Equation 5-11) for predicting crashes. This model produces accurate results and requires less data than other models. The hierarchical Bayesian process should be used for identifying segments with extreme crash frequencies that may benefit from safety improvements.

The HSM predictive model for rural two-lane two-way highways consists of a safety performance function (SPF), crash modification factors (CMFs), and a jurisdiction-specific calibration factor. For this research, two sample periods were used: a three-year period from 2010 to 2012 and a five-year period from 2008 to 2012. The calibration factor for the HSM predictive model was determined to be 1.50 for the three-year period and 1.60 for the five-year period. These factors are to be used in conjunction with the HSM SPF and all applicable CMFs.

TRB’s National Cooperative Highway Research Program (NCHRP) Report 715: Highway Safety Manual Training Materials provides training materials to aid in implementing the American Association of State Highway and Transportation Officials’ Highway Safety Manual (HSM).

"TRB's Transportation Research Record: Journal of the Transportation Research Board, No. 2515, explores 15 papers related to statistical methods and highway safety performance, including: Multivariate Full Bayesian Hot Spot Identification and Ranking: New Technique; Modeling Crash Rates for a Mountainous Highway by Using Refined-Scale Panel Data; Exploring Piecewise Linear Effects of Crash Contributing Factors with a Novel Poisson-Mixed Multivariate Adaptive Regression Splines Model; Classification of Gaps at Uncontrolled Intersections and Midblock Crossings Using Support Vector Machines; Safety Impacts of a Statewide Centerline Rumble Strip Installation Program; Evaluation of the Safety Effectiveness of the Conversion of Two-Lane Roadways to Four-Lane Divided Roadways: Bayesian Versus Empirical Bayes; Is Horizontal Curvature a Significant Factor of Safety in Rural Multilane Highways?; Developing Calibration Factors for Crash Prediction Models with Consideration of Crash Recording Threshold Change; Empirical Bayes Before-After Study on Safety Effect of Narrow Pavement Widening Projects in Texas; Transferability and Calibration of Highway Safety Manual Performance Functions and Development of New Models for Urban Four-Lane Divided Roads in Riyadh, Saudi Arabia; Safety Analysis of Freeway Segments with Random Parameters; Strength of the Variable: Calculating and Evaluating Safety Performance Function Calibration Factors for the State of Ohio; Statistical Evaluation of Different Sample Sizes for Local Calibration Process in the Highway Safety Manual; Results and Lessons from Local Calibration Process of the Highway Safety Manual for the State of Maryland; Validation Technique Applied to Oregon Safety Performance Function Arterial Segment Models." -- Publisher's description

The Highway Safety Manual (HSM) contains safety performance functions (SPFs) that are used in project-level decision-making for estimating the average crash frequency by severity level for existing conditions, alternatives to existing conditions, or proposed new roadways. However, SPF calibration is needed because most of the existing HSM SPFs were developed for states other than Texas. In addition, the HSM does not contain predictive models for frontage roads. Texas has a large network of frontage road segments that are part of the freeway system. Also, the ramp models in the HSM are not applicable to Texas due to differences in ramp configurations. Ramps in Texas usually connect the freeway mainline to the adjacent frontage road rather than a ramp terminal that connects directly to the perpendicular road, as is typical in the states used for developing the SPFs in the HSM. Researchers derived reliable local calibration factors to apply to Texas roadways for most of the SPFs in the HSM. For the facilities with poor calibration factors, researchers developed new SPFs. The calibration factors were developed by region for all facility types. Researchers developed new safety prediction models for one-way and two-way frontage roads and ramp segments. Researchers also developed an analysis spreadsheet tool to help practitioners implement the new models to facilitate analysis of all rural and urban roadway segments and intersections. In particular, the tool will assist in estimating the average crash frequency at a particular site and in evaluating different cross-sectional alternatives