> Or raise the speed limit to 65 if it's safe (probably not).
The safest value for a speed limit is what 85% of the drivers would do anyway. If everyone goes 65-70, then it's definitely safe for the speed limit to be raised to at least 65. This is exactly what they should do, and if they can't for some political reason, the next best thing would be to basically not enforce the speed limit at all on that road anymore.
This is arguably not true. We can look at the history of traffic accidents to see that, left to their own devices, drivers will drive faster than is safe. Laws, enforcement (cameras), and better road design (best), will get drivers to slow down.
The safest value for a speed limit is what 85% of the drivers would do anyway. If everyone goes 65-70, then it's definitely safe for the speed limit to be raised to at least 65. This is exactly what they should do, and if they can't for some political reason, the next best thing would be to basically not enforce the speed limit at all on that road anymore.