I recall stories told where back in the horse and buggy days you could go out to a pub and get wasted. Then get in your wagon, pass out and the horse knows the way home.
Apparently it was a common gag to switch around your mates horses. They'd end up at each others houses.
I think this r/askhistorians answer is fascinating too.
Black Beauty, published 1877, has a paragraph complaining about drunk carriage drivers and how a friend was permanently injured by one. Single horse in a small town, probably no biggie. Team of horses, not so great.
In Ohia V Miller (2024), Miller was found guilty of operating a vehicle under the influence after he drunkenly drove a horse drawn buggy. This finding was upheld my the appelete court.
In State v Blowers (1986), the Utah Supreme Court found that riding a horse while intoxicated did not qualify as DUI. This finding had 2 parts:
A) A horse is not a vehicle and
B) The provision:
Every person riding an animal or driving any animal-drawn vehicle upon a roadway is subject to this chapter, except those provisions which by their nature can have no application.
Is unconstitutionally vague.
In the case of Mythbusters v Drunk Driving, the Mybuster found that it is illegal to operate a vehicle while drunk, even on a closed course. However, it is legal for a blind man to operate a vehicle under the direction of a drunk man.
How did Charles even get the note from Jill? Did he purposely wait until he was driving to write his? How did he intend to send her his reply, was he perhaps driving over to her place to hand it over in person? Then why write it in the first place, and not just tell her!
It’s just a silly anachronistic texting while driving joke. Of course a horse wouldn’t ride off a bridge in real life when someone is writing a letter. But a car would if someone is texting.
It's not because the driver is texting (distracted), but for some reason it got scared or confused, and because it lacked the control of the carriage driver, it took a wrong turn.