National Traffic Speeds Survey III: 2015

De Leonardis, Doreen; Huey, Richard; Green, James · 2018 · ROSA P / United States. National Highway Traffic Safety Administration

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Summary

The National Traffic Speeds Survey III (NTSS III), conducted in 2015, addresses the critical need for nationally representative data on vehicle travel speeds to inform highway safety policy and speed management strategies. Since the repeal of the National Maximum Speed Limit in 1995, federal agencies lacked consistent data to track speed trends and their correlation with crash rates. Speeding remains a major safety threat, contributing to 27% of fatal crashes and billions in economic costs, with the majority of speeding-related fatalities occurring on non-interstate highways. This study, the third wave of the NTSS program initiated by the National Highway Traffic Safety Administration (NHTSA), aimed to produce accurate estimates of travel speeds for all motor vehicle types across freeways, arterial highways, and collector roads, while allowing for longitudinal comparison with data from 2007 and 2009. The study employed a stratified, three-stage cluster sampling design to ensure national representativeness. The first stage selected 24 primary sampling units (PSUs), comprising counties or groups of counties representing various regions, urbanization levels, and topographies. The second stage sampled road segments, utilizing Geographic Information Systems to oversample rare characteristics like curves and gradients. The third stage selected specific observation sites for data collection. Over 12.3 million vehicle speeds were recorded at 677 sites during the summer of 2015. Speeds on limited-access highways were measured using trailer-mounted radar devices, while speeds on arterials and collectors were captured using temporary on-road sensors. The data included both free-flow and constrained traffic conditions, with rigorous weighting adjustments applied to account for sampling probabilities and non-response. The results indicate that mean free-flow speeds on freeways averaged 70.4 mph, significantly higher than major arterials (56.4 mph) and minor arterials/collectors (49.7 mph). A substantial proportion of traffic exceeded posted speed limits: 68% on limited-access roads, 56% on arterials, and 58% on collectors. Comparing 2015 to 2009, mean speeds on limited-access highways remained statistically unchanged, suggesting drivers have reached a comfort ceiling for these environments. However, mean speeds on major arterials and minor arterials/collectors increased by approximately 3 mph, though this change was not statistically significant for means. Crucially, the 85th and 95th percentile speeds on these non-freeway roads increased significantly by 5–7 mph. Speed variation also widened on arterials and collectors, with standard deviations increasing from 10 to 13 mph. Speeds were generally lower in urban areas compared to rural ones and decreased with increased road curvature and gradient, particularly for larger vehicles. The significance of these findings lies in the divergence between freeway and non-freeway speed trends. The stability of freeway speeds contrasts with the notable increases in high-percentile speeds and variability on arterials and collectors. This trend helps explain why 87% of speeding-related fatalities occur on non-interstate highways. The increased speed variance and higher top-end speeds on these roads suggest a growing risk profile that enforcement and engineering countermeasures must address. The study provides NHTSA and state officials with robust, current data to define the speeding problem more precisely, moving beyond crash-site sampling to a comprehensive view of national traffic behavior.

Key finding

Mean free-flow speeds on major arterials and minor arterials/collectors increased by approximately 3 mph between 2009 and 2015, while freeway speeds remained stable, and the proportion of vehicles exceeding speed limits rose significantly on non-interstate roads.

Methodology

field_study

Sample size: 12330540

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• speed choice
• incidence prevalence
• speed management

Information type

What kind of knowledge this paper contributes, grouped by family — independent of topic (what it is about) and method (how it was studied).

• Empirical Findings: observational prevalence, crash risk outcomes
• Methodological Resource: dataset resource