As the old public service campaign so succinctly put it, "Speed kills." Research has shown that for every mile per hour you drive, the likelihood of your being in an accident increases by four to five percent [source: ERSO]. At higher speeds, the risk increases much more quickly.
The National Highway Traffic Safety Administration (NHTSA) explains the consequences of fast driving quite simply: "Speeding is one of the most prevalent factors contributing to traffic crashes. The economic cost to society of speeding-related crashes is estimated by NHTSA to be $40.4 billion per year. In 2008, speeding was a contributing factor in 31 percent of all fatal crashes, and 11,674 lives were lost in speeding-related crashes" [source: NHTSA].
For your average drive across town, driving even 10 mph (16.1 kph) faster is only going to save you a few minutes -- while increasing your crash risk by as much as 50 percent. Even on long trips, the time you'll save is inconsequential compared to the risks associated with speeding. Take your time and obey posted speed limits. If you really need to get there as fast as possible, there's one fool-proof solution: Leave earlier.