Do safety performance functions used for predicting crash frequency vary across space? Applying geographically weighted regressions to account for spatial heterogeneity

The number of road crashes continues to rise significantly in Thailand. Curve segments on two-lane rural roads are among the most hazardous locations which lead to road crashes and tremendous economic losses; therefore, a detailed examination of its risk is required. This study aims to develop crash prediction models using Safety Performance Functions (SPFs) as a tool to identify the relationship among road alignment, road geometric and traffic conditions, and crash frequency for two-lane rural horizontal curve segments. Relevant data associated with 86,599 curve segments on two-lane rural road networks in Thailand were collected including road alignment data from a GPS vehicle tracking technology, road attribute data from rural road asset databases, and historical crash data from crash reports. Safety Performance Functions (SPFs) for horizontal curve segments were developed, using Poisson regression, negative binomial regression, and calibrated Highway Safety Manual models. The results showed that the most significant parameter affecting crash frequency is lane width, followed by curve length, traffic volume, curve radius, and types of curves (i.e., circular curves, compound curves, reverse curves, and broken-back curves). Comparing among crash prediction models developed, the calibrated Highway Safety Manual SPF outperforms the others in prediction accuracy.

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The Development of Safety Performance Functions for Roundabouts in Amman, Jordan

MATEC Web of Conferences ◽

10.1051/matecconf/201929601006 ◽

2019 ◽

Vol 296 ◽

pp. 01006 ◽

Cited By ~ 1

Author(s):

Diana AL- Nabulsi ◽

Khair Jadaan

Keyword(s):

Urban Areas ◽

Developed Countries ◽

Safety Performance ◽

Statistical Tool ◽

Crash Frequency ◽

Road Crashes ◽

Design Variables ◽

Safety Performance Functions ◽

Strong Relation ◽

Pedestrian Crossing

It is now well established that crash occurrences at roadway segments or intersections are associated with a large variety of factors. Safety Performance Functions (SPF) are statistical models developed to predict crash frequencies for various design variables. In Jordan, almost half of all road crashes occur at intersections, mostly in urban areas. There is a significant number of roundabouts throughout Amman which is increasing without evaluating their safety performance. To assess safety benefits of this kind of intersections, transportation professionals need the powerful statistical tool; the SPF. This study aims to develop SPFs for roundabouts in Amman. The models consider the crash frequency, traffic volume and geometric features of all the studied 20 roundabouts. The developed SPFs were statistically significant (R^2 = 0.91) .The findings of the study revealed that crash frequency has a strong relation with the AADT, roundabout entry angle-degrees, entry path radius, splitter radius, pedestrian crossing structure, inscribed diameter, central diameter, circulating width, entry width, number of circle legs. The developed SPFs are evaluated through a comparison with others from developed countries.

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Development of Safety Performance Functions: Incorporating Unobserved Heterogeneity and Functional Form Analysis

Transportation Research Record Journal of the Transportation Research Board ◽

10.1177/0361198118767409 ◽

2018 ◽

Vol 2672 (30) ◽

pp. 9-20 ◽

Cited By ~ 15

Author(s):

Behram Wali ◽

Asad J. Khattak ◽

Jim Waters ◽

Deo Chimba ◽

Xiaobing Li

Keyword(s):

Unobserved Heterogeneity ◽

Random Parameter ◽

Safety Performance ◽

Validation Dataset ◽

Segment Length ◽

Significant Heterogeneity ◽

Crash Frequency ◽

Form Analysis ◽

Safety Performance Functions ◽

Functional Forms

To improve transportation safety, this study applies Highway Safety Manual (HSM) procedures to roadways while accounting for unobserved heterogeneity and exploring alternative functional forms for Safety Performance Functions (SPFs). Specifically, several functional forms are considered in Poisson and Poisson-gamma modeling frameworks. Using 5 years (2011–2015) of crash, traffic, and road inventory data for two-way, two-lane roads in Tennessee, fixed- and random-parameter count data models are calibrated. The models account for important methodological concerns of unobserved heterogeneity and omitted variable bias. With a validation dataset, the calibrated and uncalibrated HSM SPFs and eight new Tennessee-specific SPFs are compared for prediction accuracy. The results show that the statewide calibration factor is 2.48, suggesting rural two-lane, two-way road segment crashes are at least 1.48 times greater than what HSM SPF predicts. Significant variation in four different regions in Tennessee is observed with calibration factors ranging between 2.02 and 2.77. Among all the SPFs considered, fully specified Tennessee-specific random parameter Poisson SPF outperformed all competing SPFs in predicting out-of-sample crashes on these road segments. The best-fit random parameter SPF specification for crash frequency includes the following variables: annual average daily traffic, segment length, shoulder width, lane width, speed limit, and the presence of passing lanes. Significant heterogeneity is observed in the effects of traffic exposure-related variables on crash frequency. The study shows how heterogeneity-based models can be specified and used by practitioners for obtaining accurate crash predictions.

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Assessing Safety Performance on Urban and Suburban Roadways of Lower Functional Classification: A Comparison of Minor Arterial and Collector Roadway Segments

10.31224/osf.io/wgpn7 ◽

2021 ◽

Author(s):

Meghna Chakraborty ◽

Timothy Gates

Keyword(s):

Safety Performance ◽

Speed Limit ◽

General Effect ◽

Crash Frequency ◽

Mixed Effect ◽

Fatal Injury ◽

Crash Data ◽

Random Intercept ◽

Safety Performance Functions ◽

Washtenaw County

Previous research of urban roadway safety performance has generally focused on roadways of high functional classifications, such as principal arterials. However,roadways with lower functional classifications, including minor arterials and collectors, typically possess characteristics that differ from those of higher roadway classes. Therefore, assumptions made on the general effect of the predictor variables from typical safety performance functions may not apply to lower roadway classes. Toaddress these knowledge gaps, a safety performance evaluation of urban/suburban minor arterial and collector roadway segments was performed using traffic androadway data along with eight years of crash data from 189 miles of two-lane urban and suburban roadways in Washtenaw County, Michigan. Mixed-effect negativebinomial models with segment-specific random intercept were developed for minor arterial and collector road segments, considering total, fatal+injury, and propertydamage only crashes. In general, minor arterial roadways showed greater crash occurrence compared to collector roads. Posted speed limit had a significant positiveassociation with crash frequency, and this effect increased when the speed limit exceeded 40 mph. The effect of speed limit was stronger on minor arterial segmentsand for fatal+injury crashes. Additionally, driveway density was found to have a significant effect on safety performance, which was stronger for commercial/industrialdriveways compared to residential driveways and for collector roads compared to minor arterials, particularly when considering residential driveways. On-street parkingwas associated with lower crash occurrence, with a stronger effect on collector roadways, likely due to greater parking turnover when compared to minor arterials.

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Estimation of Safety Performance Functions (SPF) at signalized intersections in Medellín Colombia

DYNA ◽

10.15446/dyna.v87n214.83880 ◽

2020 ◽

Vol 87 (214) ◽

pp. 215-220

Author(s):

Víctor Gabriel Valencia-Alaix ◽

Basilio Restrepo Betancur ◽

Cristhian Lizarazo Jimenez ◽

Raul Andres Pineda Mendez

Keyword(s):

Traffic Safety ◽

Negative Binomial ◽

Signalized Intersections ◽

Safety Performance ◽

Design Criteria ◽

Binomial Model ◽

Crash Frequency ◽

Traffic Conditions ◽

Safety Performance Functions

One of the objectives of road infrastructure sustainability is to ensure that users are treated equally and their quality of life is improved by providing better mobility and traffic safety. When designing roads, it is important to evaluate different design criteria alternatives - in this case, we look at traffic safety principles. For this, we used the Safety Performance Functions (SPF) tool to obtain the expected crash frequency. The data used were Medellín’s crash records from 2012 to 2016, as well as the geometric features and traffic conditions at signalized intersections. A negative binomial model was fitted to estimate the SPF. Exposure, geometry, and traffic volume were found to be statistically significant in determining the expected crash frequency for collisions where there was property damage only (PDO) and fatal or injury (FI). It was found that accidents were less likely on T-junctions compared to four-leg junctions, one-way approaches were found to be safer whereas right turns were found to increase collisions causing FI.

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How Roundabout Entry Design Parameters Influence Safety

Transportation Research Record Journal of the Transportation Research Board ◽

10.1177/0361198118776159 ◽

2018 ◽

Vol 2672 (34) ◽

pp. 73-84 ◽

Cited By ~ 4

Author(s):

Jan Novák ◽

Jiří Ambros ◽

Jindřich Frič

Keyword(s):

Czech Republic ◽

Significant Influence ◽

Design Guidelines ◽

Safety Performance ◽

Design Parameters ◽

Crash Frequency ◽

Study Objective ◽

The Czech Republic ◽

Safety Performance Functions ◽

Safety Effect

Roundabouts are considered the safest intersection design; however, the safety effect may not be satisfactory at each specific roundabout. This is true especially in countries where roundabout design is a relatively new concept, such as in the Czech Republic. Specifically, most Czech roundabout crashes were found to occur on entries. This motivated the presented study to investigate how entry design parameters influence safety on Czech roundabouts and, if possible, use the findings to update current Czech roundabout design guidelines. To this end, the study comprised three analyses: crash-based safety performance functions, speed analysis, and finally safety performance functions which incorporated speed. All three analyses proved that entry design parameters have a statistically significant influence on safety, in terms of crash frequency, severity and speeds. Given the study objective, this fact should be considered in Czech roundabout design guidelines.

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Simple Index to Assess the Calibration Quality of Safety Performance Functions Based on Multiple Goodness-of-Fit Metrics

Transportation Research Record Journal of the Transportation Research Board ◽

10.1177/03611981211008896 ◽

2021 ◽

pp. 036119812110088

Author(s):

Raul E. Avelar ◽

Karen Dixon ◽

Boniphace Kutela ◽

Sam Klump ◽

Beth Wemple ◽

...

Keyword(s):

Goodness Of Fit ◽

Synthetic Data ◽

Calibration Procedure ◽

Safety Performance ◽

Absolute Deviation ◽

Data Set ◽

Safety Database ◽

Simple Index ◽

Safety Performance Functions

The calibration of safety performance functions (SPFs) is a mechanism included in the Highway Safety Manual (HSM) to adjust SPFs in the HSM for use in intended jurisdictions. Critically, the quality of the calibration procedure must be assessed before using the calibrated SPFs. Multiple resources to aid practitioners in calibrating SPFs have been developed in the years following the publication of the HSM 1st edition. Similarly, the literature suggests multiple ways to assess the goodness-of-fit (GOF) of a calibrated SPF to a data set from a given jurisdiction. This paper uses the calibration results of multiple intersection SPFs to a large Mississippi safety database to examine the relations between multiple GOF metrics. The goal is to develop a sensible single index that leverages the joint information from multiple GOF metrics to assess overall quality of calibration. A factor analysis applied to the calibration results revealed three underlying factors explaining 76% of the variability in the data. From these results, the authors developed an index and performed a sensitivity analysis. The key metrics were found to be, in descending order: the deviation of the cumulative residual (CURE) plot from the 95% confidence area, the mean absolute deviation, the modified R-squared, and the value of the calibration factor. This paper also presents comparisons between the index and alternative scoring strategies, as well as an effort to verify the results using synthetic data. The developed index is recommended to comprehensively assess the quality of the calibrated intersection SPFs.

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Regional Perspective of Safety Performance Functions and Their Application to Florida Intersections in Suburban Residential and Urban General Context Classification Categories

Transportation Research Record Journal of the Transportation Research Board ◽

10.1177/03611981211008447 ◽

2021 ◽

pp. 036119812110084

Author(s):

Ghalia Gamaleldin ◽

Haitham Al-Deek ◽

Adrian Sandt ◽

John McCombs ◽

Alan El-Urfali

Keyword(s):

Negative Binomial ◽

Regression Tree ◽

Influential Factors ◽

Safety Performance ◽

General Context ◽

Boosted Regression Tree ◽

Safety Performance Functions ◽

Traffic Volumes ◽

Intersection Types ◽

The Individual

Safety performance functions (SPFs) are essential tools to help agencies predict crashes and understand influential factors. Florida Department of Transportation (FDOT) has implemented a context classification system which classifies intersections into eight context categories rather than the three classifications used in the Highway Safety Manual (HSM). Using this system, regional SPFs could be developed for 32 intersection types (unsignalized and signalized 3-leg and 4-leg for each category) rather than the 10 HSM intersection types. In this paper, eight individual intersection group SPFs were developed for the C3R-Suburban Residential and C4-Urban General categories and compared with full SPFs for these categories. These comparisons illustrate the unique and regional insights that agencies can gain by developing these individual SPFs. Poisson, negative binomial, zero-inflated, and boosted regression tree models were developed for each studied group as appropriate, with the best model selected for each group based on model interpretability and five performance measures. Additionally, a linear regression model was built to predict minor roadway traffic volumes for intersections which were missing these volumes. The full C3R and C4 SPFs contained four and six significant variables, respectively, while the individual intersection group SPFs in these categories contained six and nine variables. Factors such as major median, intersection angle, and FDOT District 7 regional variable were absent from the full SPFs. By developing individual intersection group SPFs with regional factors, agencies can better understand the factors and regional differences which affect crashes in their jurisdictions and identify effective treatments.

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The variability of urban safety performance functions for different road elements: an Italian case study

European Transport Research Review ◽

10.1186/s12544-021-00490-6 ◽

2021 ◽

Vol 13 (1) ◽

Author(s):

Paolo Intini ◽

Nicola Berloco ◽

Gabriele Cavalluzzi ◽

Dominique Lord ◽

Vittorio Ranieri ◽

...

Keyword(s):

Negative Binomial ◽

Prediction Models ◽

Safety Performance ◽

Common Trends ◽

Main Research ◽

Pavement Markings ◽

Italian Case ◽

Sight Distance ◽

Safety Performance Functions ◽

Urban Safety

Abstract Background Urban safety performance functions are used to predict crash frequencies, mostly based on Negative Binomial (NB) count models. They could be differentiated for considering homogeneous subsets of segments/intersections and different predictors. Materials and methods The main research questions concerned: a) finding the best possible subsets for segments and intersections for safety modelling, by discussing the related problems and inquiring into the variability of predictors within the subsets; b) comparing the modelling results with the existing literature to highlight common trends and/or main differences; c) assessing the importance of additional crash predictors, besides traditional variables. In the context of a National research project, traffic volumes, geometric, control and additional variables were collected for road segments and intersections in the City of Bari, Italy, with 1500 fatal+injury related crashes (2012–2016). Six NB models were developed for: one/two-way homogeneous segments, three/four-legged, signalized/unsignalized intersections. Results Crash predictors greatly vary within the different subsets considered. The effect of vertical signs on minor roads/driveways, critical sight distance, cycle crossings, pavement/markings maintenance was specifically discussed. Some common trends but also differences in both types and effect of crash predictors were found by comparing results with literature. Conclusion The disaggregation of urban crash prediction models by considering different subsets of segments and intersections helps in revealing the specific influence of some predictors. Local characteristics may influence the relationships between well-established crash predictors and crash frequencies. A significant part of the urban crash frequency variability remains unexplained, thus encouraging research on this topic.

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