Nonsense

Strictly talking about safe speed in relation to other traffic, and no mention of road conditions or other factors, I had it explained to me once that speed limits are safe speeds when you're the only one on the road. When there's one other car, the safest speed is the exact same speed as the other car. You will never hit that car if you're both going the same speed (This also implies/requires safe stopping distances because you slow down with them). That's really the heart of it, whatever minimizes possible interactions with other vehicles keeps you safe. It sounds like a duh statement but its where the conversation should be based around - relative speed not actual speed. Once it's more than just you and another car, the safest speed is the average flow of traffic across all lanes. If it's rush hour and the locals are ALL doing 10-over (it definitely happens), you are being unsafe if you are driving the speed limit in ANY lane specifically because every single car on that highway passes you and each passing is an opportunity for an accident. If/when you have to pass in traffic, always try to pass vehicles no faster than 5mph faster than the other vehicle, any faster and you are the definition of 'he came out of nowhere'. That works in the other direction too - if you aren't comfortable with speeding along with the whole highway, try not to be going less than 5mph under the average speed, posted speed limit be damned.