Applying fractional split model to examine the effects of roadway geometric and traffic characteristics on speeding behavior

Afghari, Amir Pooyan; Haque, Md. Mazharul; Washington, Simon · 2018 · Crossref

DOI: 10.1080/15389588.2018.1509208

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Summary

This study investigates the influence of roadway geometric and traffic characteristics on driver speeding behavior, addressing a gap in literature that has predominantly focused on driver demographics and vehicle factors. Specifically, the research examines how site-specific factors affect not just the occurrence of speeding, but the relative magnitude of speed limit violations. The authors aim to determine whether drivers are more likely to commit minor, moderate, or major violations based on road design and traffic conditions. The methodology employs a panel mixed logit fractional split model to analyze speeding data collected from 3,765 speed cameras across 521 road segments in Queensland, Australia, between 2010 and 2013. Speeding violations were categorized into three groups: minor (less than 13 km/h over the limit), moderate (13–20 km/h over), and major (more than 20 km/h over). This speeding data was merged with comprehensive datasets detailing roadway geometry (e.g., curve radius, lane width, median type), traffic characteristics (e.g., heavy vehicle percentage, functional classification), spatial features, and driver behavioral proxies. The fractional split model was selected to account for the simultaneous occurrence of multiple violation categories at each site and to capture unobserved heterogeneity. The results indicate that drivers exhibit a baseline tendency toward minor speed limit violations regardless of causal factors. However, specific geometric and traffic variables significantly influence the severity of speeding. Wider horizontal curve radii and higher posted speed limits (≥100 km/h) were associated with a decrease in minor violations but an increase in moderate and major violations, suggesting drivers adapt their speed to road design standards. The deployment of covert speed cameras significantly reduced the proportion of major violations. Notably, the interaction between heavy vehicle traffic and divided medians was a strong predictor of major speeding; on multi-lane segments with divided medians, an increase in heavy vehicle traffic correlated with a sharp rise in major violations, likely due to overtaking maneuvers. Additionally, rural road functional classification was associated with a higher likelihood of major speeding compared to other road types. The findings imply that road safety countermeasures should be tailored to specific geometric contexts. For instance, traffic calming measures may be effective on wide-radius curves, while exclusive lanes for heavy vehicles could mitigate overtaking-induced speeding on multi-lane highways. The study also suggests that covert enforcement is more effective at reducing severe speeding than overt methods. These insights provide a basis for designing targeted interventions that address the specific roadway and traffic conditions driving different magnitudes of speed limit violations.

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Topics

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• speed choice
• perceptual countermeasures
• speed management
• road geometry
• rail grade crossings

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• Empirical Findings: observational prevalence