The point of this article is not that speeding is safer, but that wide variance in speed create their own risks, and some drivers will obey a posted speed limit

The point of this article is not that speeding is safer, but that wide variance in speed create their own risks, and some drivers will obey a posted speed limit that is lower than the road mean. OTOH, many surface streets necessarily have wide variances in vehicle speeds because they serve varied vehicle users or other reasons, and wide variances in speed are more likely to be cause by speeding cars.