The legal limit might be 70mph, but when traffic is not heavy it typically flows much faster than that in my experience. I used to make a regular commute along I95 through Delaware, and even though the speed limit was 55 (later upped to 65), it wasn't uncommon to be going 75-80 alongside other cars. Not everyone went that fast, but most people seemed to. Note that the speed of traffic hardly changed when the limit was increased.