The cause was easy enough to identify: Data parsed by Kuhls and her colleagues showed that drivers were speeding more, on highways and on surface streets, and plowing through intersections with an alarming frequency. Conversely, seatbelt use was down, resulting in thousands of injuries to unrestrained drivers and passengers. After a decade of steady decline, intoxicated-driving arrests had rebounded to near historic highs.
… The relationship between car size and injury rates is still being studied, but early research on the American appetite for horizon-blotting machinery points in precisely the direction you’d expect: The bigger the vehicle, the less visibility it affords, and the more destruction it can wreak.
Hmm, they’re pretty synonymous, but I think you’re noticing this slight, occasional difference in use: Lethal is active, deadly is passive. A thing can actively be lethal when used by you, but when it’s something that happens to you, it’s deadly. An accident is something that’s considered to have happened to you, despite the fact that it’s typically your fault to some extent.