This  paper exploits the dramatic increase in tickets during the Click-it-or-Ticket campaign to identify the causal impact of tickets on accidents using data from Massachusetts. I find that tickets significantly reduce accidents and non-fatal injuries. However, there is limited evidence that tickets lead to fewer fatalities. I provide suggestive evidence that tickets have a larger impact at night and on female drivers.
That is from the abstract of Dara N. Lee's paper.
Fascinating, especially because of the implications for most developing countries where speed limits are generally not enforced.