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Examining the impacts of crash data aggregation on SPF estimation.
Agnimitra Sengupta1, Vikash V Gayah1, Eric T Donnell1
1Department of Civil and Environmental Engineering, The Pennsylvania State University, 231 Sackett Building, University Park, PA 16802, United States.
Accident; Analysis and Prevention
|August 8, 2021
Summary
Estimating safety performance functions (SPFs) using aggregated crash data can lead to biased results and inconsistent Empirical Bayes adjustments, potentially impacting road safety network screening. Disaggregate data offers a more reliable approach for accurate safety analysis.
Area of Science:
- Transportation Engineering
- Traffic Safety Analysis
- Statistical Modeling
Background:
- The Highway Safety Manual (HSM) utilizes safety performance functions (SPFs) for road safety assessment.
- SPFs are typically estimated using negative binomial regression with either aggregate or disaggregate crash data.
- The choice of data aggregation impacts the reliability of safety estimations.
Purpose of the Study:
- To investigate the differences in SPF estimation using aggregate versus disaggregate data.
- To evaluate the impact of data aggregation on Empirical Bayes adjustments and network screening.
- To compare SPF estimation methods used in the HSM with aggregate and disaggregate datasets.
Main Methods:
- Utilized negative binomial regression modeling, consistent with HSM methods.
- Employed a synthetic dataset generated with negative binomial distribution properties.
- Analyzed an observational dataset from Pennsylvania comparing aggregate and disaggregate data.
Main Results:
- SPF model coefficients showed similarity between aggregate and disaggregate data.
- Significant differences were observed in the overdispersion parameter between the two data types.
- Biases in expected crash frequency calculations and inconsistent Empirical Bayes adjustments were identified.
Conclusions:
- Aggregating crash data can introduce systematic biases in SPF outputs.
- Differences in overdispersion parameters affect Empirical Bayes adjustments and network screening outcomes.
- Disaggregate data provides a more robust basis for SPF estimation and safety management.
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