Don't know if it's fact but it's generally accepted that when people marry they take on the additional responsibility -- if not maturity -- of providing for another person's physical and emotional well-being. Most people, when they begin living for something other/greater than themselves, behave in more .... "thought-out" .... ways. Not that they were reckless before, but now thay think twice before turning left without a green arrow light when the opening between oncoming cars is a little short. Now they think twice before pulling onto a busy main road in front of approaching traffic from a stop at an intersecting residential outlet. Now they actually do go a little slower in the rain.
Although anyone who's unexpectedly lost traction a few times in the rain would do this anyway.
Granted, some of it is more age than marital status but a 40-year-old bachelor isn't necessarily going to have a little angel sitting on his shoulder reminding him that his little girl really would like to see him again tonight whereas a 27-year-old Navy PO-1 has that little angel sitting on his shoulder reminding him every second of every day. You're a safer person all around when you have someone to live for beside yourself. And that changes the game.
Remember, with insurance companies, you're in a gambling game. You're gambling that you'll get in an accident. The InsCo is gambling that you won't. If you don't, they win and you lose. You pay them their winnings. If you do get in an accident, you win, they pay you your winnings, which are higher because the odds are in favor of you not being in an accident. If you're suddenly a lot safer because you now have someone to live for, that changes the odds, and the game, and they have lower winnings when you don't have an accident.