The National Academies Press has issued an 80-page report entitled "Guidelines for Nighttime Overhead Sign Visibility." It includes a chart that's headlined "Luminance Levels for Overhead Signs." It lists five different visual complexity levels, ranging from a dark rural area to a commercial downtown district. It then suggests minimum luminance levels in terms of candelas per square foot and square meter. The suggested brightness levels at the most complex setting are 10 times higher than for the rural setting.
You can download the entire report at https://www.nap.edu/download/23512