Driving at a higher than reasonable speed increases your risk in two ways: it cuts your reaction time and results in more "stored" energy (that must be dissipated in any collision). You should consider if the risks are worth the gain.
This is the science of math and physics—you cannot bend these rules. Each incremental increase in speed reduces your ability to react in time to hazards, because you may be covering distance in less time than it takes to react. Normal reaction time is between .75 second and 1.5 seconds, on average. Average reaction time distance at 50 mph would be approximately 83 feet. At 70 mph, it is over 115 feet (over 7 modern car lengths). These numbers do not include braking distance, just reaction time. The average difference in reaction-time distance from 50 mph to 70 mph is about 32 feet. If you were relying solely on braking, any hazard you encounter within the reaction distance is already a problem; you can't react quickly enough to miss it. This is particularly important at night, when darkness restricts your visibility. Do you know at what distance your headlights will illuminate a hazard? How is your night vision these days? When headlights finally light up a road hazard, it is often too late to avoid it. Many experts would tell you that even 50 mph is too fast for conditions at night, on any dark roadway.
If you could choose the speed at which to hit a brick wall, assuming that it was a sure thing you were going to hit one, would you choose to hit the wall at 10 mph or at 100 mph? Not hard to decide, is it? Higher speeds also bring additional accumulated, or stored, energy. More stored energy means increased crash forces if you hit something. Here's a real-world example; a loaded semi traveling at 60 mph develops about 6.5 MILLION foot-pounds of force. Or, your body, unrestrained in the vehicle, could hit the windshield with about 16,000 foot-pounds of force, should your vehicle hit some immoveable object - like a tree.
A defensive driver chooses a speed matching traffic as closely as possible without exceeding speed limits. If traffic is moving at higher speed than you should go, keep to the right and out of the way. This is often a legal requirement as well, if you are traveling at a speed less than the flow of traffic. Also, don't neglect to maintain the correct following distance.
Consider that speeding often doesn't save much time. How many times have you reached a red light, only to find a "jackrabbit" waiting there that passed you a half mile back like you were standing still? Ever wonder why? Around most urban areas, signals limit overall speeds to what the system can handle (in terms of numbers of vehicles). In Phoenix, for example, that's approximately 40 to 45 mph. Drive faster than that and you'll simply spend more time waiting at red lights, wasting fuel, wearing down brake pads, and accumulating just a little more stress in your life for no good reason or gain. Even on the highway, you don't often gain much. Frequently, once you pass someone, you find them on your back bumper as you slow down to enter the next town. So you gained what, exactly? On an Interstate, where you truly can save some time by speeding (provided you don't get pulled over), the difference between 65 mph and 80 mph over 50 miles is only 8.7 minutes. Big deal.