I ran a quick Google search and found this for trains:
http://safetydata.fra.dot.gov/officeofsafety/ <-- this is obviously only relevant to the United States (as far as I know), but rather credible given that the website is maintained by the federal government
If you search for "Motor Death Statistics", you will find a variety of sources, but you will need to take the time to assess which ones are credible or not.
To give an example:
http://www.the-eggman.com/writings/death_stats.html <-- this website has motor death statistics for 2002 in the United States, but I am reluctant to trust the information it gives because I know nothing about the website. The source it lists is essentially this (although an earlier version):
http://www.cdc.gov/nchs/products.htm#nvsr <-- but I believe you have to pay money to access reports from their website, if you have access to a university library, they may have the report available for free.
If you want to take into account statistics for countries outside the United States, you may want to ask a librarian about what reference books to consult. Additionally, playing around with Google search terms can often lead to hits regarding the information you want - that is how I was able to find data on US trains.
I hope you find what you need. If I have time, I might be able to discover more information.