Safety Performance Functions (SPFs) in the Highway Safety Manual (HSM) were developed using historic crash data collected in different states. To make the SPFs better accommodate the local data, two strategies are usually undertaken: the first strategy is to calibrate SPFs provided in HSM so that the contents of HSM can be fully leveraged and the second strategy is to develop location-specific SPFs regardless of the predictive modeling framework in the HSM.
Calibration & Development of Safety Performance Functions for New Jersey
Overview
Research Objectives
The main objectives of this research project are:
(1) Calibrate the SPFs provided in the HSM using New Jersey (NJ) data
(2) Develop new NJ-specific SPFs as appropriate.
The facility types considered for this research project include segments
and intersections of rural two-lane two-way, rural multilane, and urban and suburban
roads. The following tasks were completed to achieve the main project objectives:
- Conducted an in-depth review of the relevant studies in the literature
- Identified the key sources of data required for calibration and development of SPFs. These include roadway characteristics data, traffic volume data, and crash data.
- Developed a computer code to read and process the compiled database to (a) filter out inconsistent data entries, (b) identify facility types, (c) execute roadway segmentation process, (d) assign crash statistics for each facility, and (e) generate a complete database for each facility type to be used in calibration and/or development of SPFs.
- Provided recommendations to improve data collection and recording practices that would facilitate easier data extraction required for the SPF calibration/development process.
Deliverables
Using the gathered and processed New Jersey specific data the research team estimated calibration factors for 13 facility types, including rural two-lane two-way segments and intersections, rural multilane highway segments, urban and suburban segments and intersections. The Calibrator tool developed by the FHWA was utilized to calculate the calibration factors and measure their goodness of fit. Using the same data used for calibration, the research team developed NJ-specific SPFs for 11 facility types, including rural two-lane two-way segments and intersections, urban segments and intersections with a sufficient number of data points using the Voyager Safety crash data from 2011 to 2015. SPFs were estimated based on the negative binomial model suggested by the HSM. The team used the R statistical package to estimate the model parameters and their statistical significance. The research team also modified the spreadsheets developed by the HSM, which are being currently used by the NJDOT staff. The calculated calibration factors and the developed SPFs are embedded in these spreadsheets. The users can now select whether to use the HSM SPFs with the calculated calibration factors or to use the New Jersey specific SPF in their analyses.
