For most types of road equipment there are regulations that set performance requirements: a noise screen should mute the sound to a maximum permitted level, a road marking should have a minimum retroreflectivity for it to have sufficient visibility, etc. With regard to street lighting there are multiple functional requirements described in the Swedish Transport Administration's publication Requirements for roads and street design (VGU), including requirements for allowed glare, demands on shielding, illuminance and road surface luminance. On most roads and streets are luminance requirement is the most important parameter. As an object on the roadway almost always appears darker than the background (the road surface), a bright road gives a high contrast and therefore good visibility, which is important, especially for the detection of pedestrians. For a requirement in the regulations to be meaningful, it is important that it is possible to check if the requirements are met. However, luminance measurements are complicated and susceptible to interference from ambient light sources. To measure the luminance with a conventional photometer is certainly possible, but extremely time consuming. A CCD camera can be useful, but the measurements must be made in the off-peak hours, without glare from oncoming vehicles, which would give rise to systematically too high luminance. There is therefore a need for an instrument that can carry out fast mobile measurements of the road surface luminance without the risk of interference from other traffic. Such an instrument would permit measurements, similar to those made on road surface profile and visibility of road markings, to be performed. The purpose of this study was to develop a prototype of an instrument for mobile measurement of road surface luminance in street lighting. The measuring instrument consists of a trailer on which it is mounted three photocells (lux meters), an optokator and a data collection device. Of the three photocells the central one is fixed and centered with respect to the trailer width. The two outer photocells are mounted on foldable arms, so that the lateral position across the driving lane can be varied, allowing for measurement in the positions specified in SS-EN 13201 — Road lighting - Part 3: Calculation of performance. Completed tests with the instrument have shown that the measurement of road surface luminance on longer sections can be performed, without light interference from other traffic. The measurements are also relatively rapid and requires a minimum of manual data processing. The measurement system additionally has good accuracy which makes it suitable for condition assessment of road lighting. (Author/publisher)
Abstract