Continuous freeway illumination and accidents on a section of Rte. I-95.

Hilton, Marvin H · 1978 · ROSA P / Virginia Transportation Research Council

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Summary

This study evaluates the effectiveness of continuous freeway illumination in reducing nighttime accident rates, specifically addressing concerns raised during the 1973–74 energy crisis when lighting was curtailed to conserve resources. The research focuses on a 13.8 km (8.57 mi) section of Interstate 95 in Virginia between Springfield and Washington, D.C. The primary motivation was to determine if the energy savings from turning off lights justified the potential increase in nighttime accidents, building upon a preliminary three-month study that suggested lighting reduced accident rates. The methodology involved a comparative analysis of accident data from two six-month periods: December 1972–May 1973 (lighting on) and December 1973–May 1974 (lighting off). The study utilized a refined analytical approach, calculating night-to-day accident rate ratios per million vehicle miles of travel (MVMT). To ensure accuracy, traffic counts were adjusted to define "night" as the period from 15 minutes after sunset to 15 minutes before sunrise, accounting for natural light decay. Statistical significance was tested using conservative and liberal curves based on Poisson distribution and chi-square tests to determine if observed reductions were due to chance. The results indicated that continuous lighting significantly reduced the nighttime accident rate. For the winter period (December–February), when traffic volumes were high and darkness prolonged, the lighting reduced the night-to-day accident rate ratio by 44%, a finding significant at the 95% confidence level. During the spring period (March–May), the reduction was 10%, which was not statistically significant due to the low number of accidents. Over the entire six-month period, the lighting reduced the night-to-day accident rate ratio by 36%, a result significant at the 95% confidence level. The study also noted that while total accident numbers decreased during the "lights off" period, this was largely attributed to reduced traffic volumes and lower speed limits during the energy crisis, rather than the absence of lighting. The study concludes that continuous freeway illumination is effective in improving safety by reducing nighttime accident rates, particularly during winter months with high traffic volumes. It recommends that any future plans to reduce energy consumption by turning off freeway lighting should be carefully weighed against the adverse effects on safety. The findings support the operational value of lighting in enhancing visibility and reducing accident severity, despite the high costs of installation and maintenance.

Key finding

Continuous freeway lighting reduced the nighttime accident rate by 36% over the six-month study period, a result that was statistically significant at the 95% confidence level.

Methodology

naturalistic

Sample size: 190

Provenance

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

Topics

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• roadway lighting effects
• visibility analysis litigation

Information type

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• Empirical Findings: crash risk outcomes