【国外标准】 Standard Practice for Evaluating Nighttime Retroreflective Sheeting Effectiveness Using Positive-Contrast Textual Sign Scenario Performance Analysis

5.1 Retroreflective sheeting is commonly used to improve the nighttime visibility and legibility of traffic signs under vehicle headlight illumination. This standard provides a procedure for evaluating the nighttime retroreflective performance of sign sheeting used in roadway signing in terms of an overall average performance index for predefined road scenarios.5.2 A procedure to characterize the relationship between sign luminance supply and driver luminance demand at night without conducting field work helps traffic engineers and transportation agencies responsible for specifying highway construction materials and maintaining roadway safety in making informed decisions about the performance of retroreflective sheeting on the signs. The procedure requires the comprehensive measurement of the retroreflective properties of a sheeting according to Practice E809 over a wide range of angles.5.3 A variety of retroreflective sheeting is available for use on traffic signs. Coefficients of retroreflection are typically measured for a standard set of angle combinations and are used, in part, to certify conformance to a specification or standard. However, while coefficients of retroreflection on some standard angle sets can provide a general idea about a sheeting’s retroreflectivity and help certify conformance to a standard, a more comprehensive analysis is needed to determine how well a sheeting is expected to serve drivers in general, or in specific use-cases. Drivers of different vehicles viewing a multitude of signs in the real world experience a much more complex set of angular combinations than those captured in the standard angle sets. Furthermore, drivers observe luminance, which is affected by not only the coefficient of retroreflection, but also by headlight illumination, distance from the vehicle to the sign, and light attenuation, among other factors.5.4 This practice utilizes a set of driver sign viewing scenarios. When combined with the coefficients of retroreflection at the corresponding geometry, the luminance as observed by the driver is calculated. The luminance requirements of the driver for varying percentiles are also tabulated for the user. Comparing the luminance supply from the sign with the luminance demand of drivers provides an assessment of the expected “performance index” in each scenario.5.5 The data on the luminance needs of the driver represents the visual performance of a subset of legal drivers in the United States of 55 years of age or older and may not represent the visual performance of the entire driver demographics.1.1 This practice provides a framework to evaluate retroreflective sheeting performance in nighttime driving conditions without a need for field evaluations through a set of sign viewing scenarios representing common use-cases. The evaluation of performance of a specific sheeting is achieved by comparing the luminance provided by a sheeting to the luminance needed by drivers in each scenario. This comparison is expressed in terms of a “Performance Index,” which is a measure for how well the luminance provided to the driver meets their needs, in each of the scenarios. Comparison of the performance index values for different sheeting allows the user to predict differences in nighttime retroreflective performance when those sheeting are used on installed signs.1.2 The driver-needs data is based on textual signs (not on symbolic signs) with positive contrast (sign text being brighter than its background), and the headlamp illumination is assumed to be low-beams; therefore, performance index is applicable only to textual signs viewed under low-beam headlamp illumination.1.3 Units—The values stated in either SI units or inch-pound units are to be regarded separately as standard. The value stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the standard.1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.5 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.