Visibility of flat line and structured road markings for machine vision

Abstract

Road markings are equally needed for human drivers and for machine vision equipment; their visibility demands high contrast ratio. Particularly difficult is achieving visibility under the conditions of wetness at night and in the presence of glare from an oncoming vehicle. A field evaluation of visibility by camera and by LiDAR was done on two types of road markings that were applied as pedestrian crossing: flat lines with typical retroreflectivity and structured lines with high retroreflectivity – thus, extreme cases were assessed. The measured camera contrast ratio dropped meaningfully in the presence of moisture in case of flat line markings, but remained high in case of structured markings that facilitated moisture drainage. Glare was detrimental to visibility, bringing it to almost naught regardless of the type of road markings. Simultaneous evaluation with LiDAR showed profound differences under the conditions of moisture: while the response from flat line markings dropped to nil (recovery time >20 s after wetting), the structured markings continuously provided meaningful response. This outcome proves that for reliable guiding by machine vision, as well as for human drivers, structured road markings that facilitate water drainage should be used. For dependable steering by machine vision equipment under adverse conditions, a combination of LiDAR and camera is seen as necessary.