Does anyone know if (and where) it's possible to download a data set of all car accidents (or just fatals) for the last few years? I'd like to see things like date and time, weather conditions, lat/lon, make and model of cars involved, etc.
All California crashes, fatal to no injury, goes back a few years at least: http://iswitrs.chp.ca.gov/Reports/jsp/userLogin.jsp (You have to create a free account)
Overall these data sets have been pretty well studied for the past few decades. The results are well documented: e.g. 6pm to midnight is the deadliest time, young males are the riskiest, older drivers are also risky, etc. Weather can actually be good for safety because people often drive slower and there are fewer cars on the road in bad conditions. But yes, weather can be a factor too.