If you drive ten miles above the speed limit since you're late, you likely believe that you're saving time. But at 70 miles an hour, it takes 51 seconds to travel a mile. And if you go 80, it takes 45 seconds, so you're just saving 6 seconds per mile. Now, you'll also reach slower cars sooner, which means you'll hit the brakes more a bit. You'll also change lanes, and you'll need to be more careful overall. And these small delays build up, and can quickly cancel out those few seconds you gained