We've all seen billboards saying "Speeding Kills". But why does speeding kill? Most people would say if you drive fast the impact of the collision will be greater, and the greater the collision the greater the chance of dying. This is true. Another reason why speed kills is the faster you travel the less you see. So as your speed increases you ability to scan for hazards efficiently decreases. Lets see an example...
Driving at 50 mph means you are moving close to 75 feet per second. Increasing your speed to 60 mph means you are moving close to 90 feet per second. Human vision is only high definition in an extremely small area. Outside this area our vision becomes blurry (as in peripheral vision). So if your traveling at 60 mph and you look 45 degrees to your right for 1 second - almost everything to your left becomes blurry for over 90 feet of travel - so by the time your eyes scan 45 degrees to the left you will have traveled over 90 feet before your high definition vision picks up on the hazard - lowering your reaction time which means you will hit the brakes harder or swerve more (sometimes we do both to avoid a wreck). If you would have stayed at 50 mph you would have seen 15 feet more viewing area. 15 feet could mean life or death.
The higher the speed also means the more stopping distance required to stop safely. The table below is from an older version of the Idaho Drivers Manual. Take a look at how increases in speed drastically increases stopping distance (braking distance). Using the example above increasing your speed from 50 mph to 60 mph increases your stopping distance 61 ft! Notice in the small box it says "Total minimum stopping distances with perfect 4-wheel brakes on the best type of road surface under favorable conditions." The concept of stopping distance is extremely important in regards to following distance. Most drivers do not increase their following distance as their speed increases - or they don't increase it enough. (It's also usually hard to maintain following distance if your speeding.) Take a look at the Thinking Distance (I call it reaction time distance). This is why speeding and tailgaiting are a very bad combination (Remember The Big 6 that cause wrecks - Speed, Space, Distraction, Visibility, Traction, and Impatience. Tailgaiting is a space issue that you have lots of control over).
As a good driver you will be able to determine if a speed limit is too low or too high. On a trip in Oregon I noticed a major road going inland from the coast was dangerously too fast. My scanning ability was greatly compromised because at the speed limit my eyes were glued to the road itself - I could not look anywhere else and to take a quick glance at my tailgaitor was not safe either. Yet the main highway going through southern Oregon to Idaho was 55 mph - painfully too slow (a vehicle passing us going 60 mph was pulled over a few miles ahead of us!). When determining what speed is safe you must ask yourself "Is my scanning suffering because of my speed?". If you feel that you are missing things because of your speed - please bring it down a few mph. The faster you travel the more likely you will miss something.
So in summary speeding is not just about hitting things hard, it increases your chance of hitting something hard because your eyes aren't quick enough to see everything, and if you do see something you may not have enough stopping distance to avoid it.
Speed limits are set based upon many factors. This is copied from the Idaho Transportation Department...
How are speed limits set?
Speed limits are intended to supplement the drivers’ judgment in determining what is a reasonable speed for particular road and weather conditions. Limits are imposed to assist Idaho law enforcement. They encourage better traffic flow by reducing the variances in speed.
Traffic limits that reflect the behavior of the majority of motorists are found to be successful. Laws that arbitrarily restrict the majority of drivers encourage wholesale violations, lack public support, and generally fail to produce desirable changes in driving behavior.
In accordance with federal guidelines, ITD uses the 85th percentile speed of free flowing traffic for determining a safe and reasonable speed for a given road section. Traffic Engineers set the limit at the speed at which 85 percent of the traffic is driving. This reflects the safe speed as determined by a large majority of drivers.
According to research, accident involvement is the lowest within that 85 percent. Speed limits are also determined by a combination of two investigations involving engineering and traffic:
The engineering investigation involves determining the design of the road and its immediate environment. Engineers analyze such items as lane width, pavement type and condition of the road. They also look at terrain, parking conditions, residential development along the road and the number, width and types of entrances and intersecting streets.
The traffic investigation involves gathering and analyzing traffic related data such as traffic volumes, accident frequency, and the effect of traffic control devices such as stoplights and stop signs.
After all variables have been considered and a speed limit is established, traffic should flow at a safe and efficient level.
They also look at the amount of pedestrian, bicycle, and vehicle traffic. Center turn lanes also affect speed limits. Many roads are 5 mph slower if there is no center turn lane. Construction areas are also slower because of less lanes, smaller lanes, and less escape options (concrete barriers that prevent right swerving).