Redefinition of Energy Efficiency and Utilization Coefficients for Human-Centered Lighting: A Must for Urban Sustainability

The main target of street and road lighting is to ensure the safety and the well-being of pedestrians and drivers. Regulations and standards on lighting installations establish minimum photometric requirements to achieve it. Thus, the main parameters concern the average luminance or illuminance, overall uniformity, longitudinal uniformity, threshold increment, edge illuminance ratio, and minimum energy efficiency or its equivalent. Although they have well-defined minimum and maximum values, their compliance, especially in urban and peri-urban environments, strongly depends on the heterogeneous characteristics of the street and its surroundings, depends on human physiological and psychological aspects, and/or faces remarkable uncertainties and problems of definition. The coefficient of utilization, Cu, and energy efficiency, ε, are key quantifiers taking account of the installation capability to provide luminous flux on the visual work plane with respect to the flux emitted and the power consumed by the light sources, respectively. However, contradictions between accurate values of these coefficients and the real visual performance of people or the rational use of energy are frequent. This is a problem because the binomial safety–sustainability requires consideration of these parameters in the design to enhance pedestrian and driver safety, as well as energy efficiency for sustainability. This work highlights the uncertainties and limitations of Cu and ε, redefines them through a human-centered approach that opens new perspectives on other parameters and quantities involved in lighting, and optimizes the binomial safety–sustainability.