MKE DNS said that this gravel lot behind my duplex couldn't be used so I had to block it off.   For this, I used rail road ties secured with 1/2" re-bar.  3 pieces, each 3' deep.

2 weeks later, my tenants told me that they were woken up at 3 am by a drunk Mexican (their words) that hit the tie so hard that his front tires went over it and were in the air.  He had no way to back up and thought that sounding his horn until someone came to his aid was his best option.