Because your road is private, you will probably have difficulty getting town police to enforce your speed limits. We have found driving habits hard to change. Is there a specific member (or members) close to the offender who could intervene? Try not to escalate the situation. 

Our association uses a combination of cajoling, good-natured signage, and nautical manila "speed ropes". (See photos attached) The fabrication of these speed ropes was originally described in our news letter,  MUDSEASON 2020. I have recently revised the original and added "Nautical Manila 'Speed Ropes' for Traffic Control" to the Resources Page on our website.

I collected all the seasonal materials on our private road yesterday for winter storage and have just ordered the synthetic manila material suggested in the reference above for some replacements in the spring!

I'm also trying out a biodegradable cotton nautical rope:

https://sgtknots.com/collections/rope/products/twisted-100-percent-cotton-rope?variant=33464447729750