Just adding some experience : my in law family lives in a rural area, next to a road where the cars are certainly few - but certainly go very fast too (it wasn't a village so no speed limit, and even though there now is a limit, it isn't respected...).
My boyfriend has had something like 13 cats when he was small (the cats weren't allowed inside a lot, they were for catching mice and so on), and they almost all got run over by a car. Now there's still 2 cats but those were allowed inside when they were younger (they're at least 6 and 7 years old now as I knew them from the start, mother and son
) and have grown car wise I guess - but I'd also say that the son must have FIV and/or FeLV as he's not neutered and always fighting (even foxes we guess
).
And also, some of the old fashioned neighbours (that's to say, about all of them) seems (seemed ?) to think it fun to try to kill the cats by running over them with farm machines (while harvesting etc).
So yes, of course some cats may live to a long age - but how many will die young of a car accident or from FIV (no vaccine !) or FeLV ?
YES there are studies that have been made to assert that the life expectency of an outdoor cat (especially not neutered as he will put up some fights and may be infected by FIV) will be divided by 2...
Now this is your choice, but I know that 5 years ago I would't have dreamed of NOT letting my cats free (I never had any then and I wasn't a vet) - now I'm just thinking the contrary (which my mother doesn't understand but she'll have to live with it
).