"Every death on every road in Great Britain 1999-2010" [bbc.co.uk] by the BBC shows the accumulation of 2,396,750 unique crashes that occurred between 1999 and 2010, on a single, beautifully rendered map. It also contains a small series of animated time-lapse movie that aggregates the crashes per hour and per weekday. The intensity of brightness shows where road collisions have been more frequent.
The BBC also offers alternative means to explore the data, such as a few revealing infographics (showing that most pedestrian crashes happen around school home time; bikers account for 1% of all accidents, yet leading to 21% of all fatalities, etc.) and an interactive map to allows for investigating neighborhoods of choice.
The maps seem to be based on the same dataset that also drove Road Casualties UK 2000 - 2010 by ITOWorld.
See also Britain Seen from Above.