My guess would be that whoever lives on that driveway makes frequent left-hand turns coming out of their driveway and they petitioned the count/city to reduce the speed limit 500ft before and after their driveway.
I know that in Texas the locals on a street can petition to have the speed limit set however lower they want it but cannot set it above state and city maximums.
I used to drive down a 4-lane divided street, better than some state highways, for my commute and the locals "got tired of those speedster kids" and had the speed limit set to 30 mph (not a typo).
You'd be amazed what you can do if you go down to the local courthouse and filed a few zoning petition docs. Sure they go thru a grace period where they can be contested and refused, but how many people go to the courthouse to see if anyone has put in for crazy petitions that need some refusing brought on'em?