Mostly, some date back to the 1800s. Some are a bit more modern, but still mid 1900s. Though in farm country it isn't unheard of to save money buy not building one up to the weight of a loaded truck. For that matter, there are farmers who run more than max weight of the road (this is sometimes even legal, it damages the roads, but replacing a road a bit early is considered cheaper than building to higher capacity - bridge weight limits cannot legally be exceeded though, and farmer going overweight know where they can't go)