Ever contemplate why driving at twilight is difficult? In Visibility of SMV Signs during Twilight, visibility expert witness Raymond L. Lee, Jr., Ph.D.,writes:
During the darker half of civil twilight, low levels of natural illumination make slow-moving-vehicle (SMV) signs less visible. Their visibility decreases then because drivers must rely more on luminance contrast than color contrast, and a SMV sign’s fluorescent orange actually reflects less luminance than does a white surface. Furthermore, about 10%-15% of drivers do not use their headlights during twilight’s darker half, and this behavior renders ineffective the red retroreflective edge of the SMV sign. My spectroradiometric measurements show that adding a white border (reflectance = 90%) to the SMV sign would make its contrast exceed the threshold contrast (and thus make the sign detectable) during more of twilight, even for unalerted drivers who do not use their headlights. This added margin of safety for farmers, farm workers, and motorists suggests a simple, but significant, improvement to current SMV sign design.