There are literally hundreds of studies that demonstrate that higher speeds are correlated with a larger number of car accidents. This is particularly true in crowded urban environments and general access highways.
The other big issue regarding speeding in urban environments is that not only does it increase the risk of accident, but it increases the impact of the accident as well -- the energy involved in a 60 mph crash is double a 40 mph crash. So when some idiot drives 60 mph on a sidestreet and hits a parked car, he's more likely to hurt himself and other and will likely cause much more damage.
The top 6 causes of US car accidents are, in order: distraction (including rubbernecking), fatigue, drunk driving, speeding, aggressive driving and weather.
Also, speed limits have minimal or no measurable impact on traffic density. In the US, traffic density is generally a byproduct of too much road infrastructure. Read about the impact of the Triborough Bridge on NYC traffic in the 1930's for an illustrative example.