Experimental traffic control device testing at New Hampshire toll plazas.

NHTSA · 2007 · ROSA P / New Hampshire Department of Transportation

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Summary

This report evaluates the effectiveness of experimental purple lights installed as supplemental traffic control devices at the Hooksett Toll Plaza on Interstate 93 in New Hampshire. The study was motivated by safety and operational concerns regarding last-minute lane changes and weaving maneuvers near toll booths, which create conflicts between E-ZPass users and cash-paying customers. Existing guidance, primarily overhead signs and green signal lights, was found to be insufficient, often attracting cash-paying drivers into dedicated E-ZPass lanes. The Hooksett site was selected due to its challenging horizontal and vertical geometrics and susceptibility to early morning fog, conditions that may benefit from enhanced wayfinding lighting. The experiment, conducted in October 2006, involved installing purple LED lights above dedicated E-ZPass lanes (Lanes #4 and #5) accompanied by advanced signage. The study design compared "before" conditions with two "after" conditions: steady (solid) purple lights and flashing purple lights. Data collection included field observations of lane changes and toll violations, as well as a customer survey. Four Measures of Effectiveness (MOEs) were analyzed using statistical tests of proportions to determine significance at a 95% confidence level. These MOEs assessed lane changes by E-ZPass customers, lane changes by cash-paying customers, utilization rates of dedicated E-ZPass lanes, and the frequency of toll violations. Statistical analysis revealed that the purple lights significantly altered driver behavior. Both steady and flashing purple lights reduced the occurrence of cash-paying customers changing lanes from dedicated E-ZPass lanes into adjacent cash lanes. Additionally, both light modes reduced the percentage of E-ZPass customers utilizing cash lanes that also accepted E-ZPass transactions. Regarding safety, the steady purple light significantly reduced toll violations in Lanes #3, #4, #5, and #7, while the flashing purple light reduced violations in Lanes #3, #4, and #5. The study noted that these behavioral changes were small relative to total traffic volume but statistically significant. A survey of 219 E-ZPass customers indicated strong acceptance of the device. Approximately 51% of respondents used the lights for guidance, and 67% found them helpful. These figures increased to 70% and nearly 80%, respectively, under poor lighting or inclement weather conditions. Among those who found the lights helpful, 67% preferred the flashing mode. The report concludes that the purple lights improved operations without negative impacts and recommends further testing of modifications, including flashing yellow lights, increased LED intensity, back plates for visibility, and larger overhead signs to maximize effectiveness.

Key finding

The implementation of purple lights significantly reduced the occurrence of cash-paying customers changing lanes into dedicated E-ZPass lanes, decreased the percentage of E-ZPass customers in cash lanes, and lowered toll violation counts in specific lanes.

Methodology

field_study

Sample size: 219

Provenance

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

Topics

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• emergency work zone conspicuity