Arlington County Parks and Rec responded to our inquiry:
"Thank you for writing about electric bikes on Arlington County Trails. The current County Code 14.2 - 64.1 section B stipulates that electric motors among other things are prohibited. I have attached the excerpt for your review. Perhaps electric cycling and other technologies were not common at the time the code was written. It is however the current policy."
Really? Wonder how they will be enforcing that. Once again, traffic law is disconnected from the cyclists that use the transportation infrastructure. #scofflaws
Here is an interesting note from electric-bikes.com:
In 2001, the U. S. Congress passed Public Law 107-319 which exempts electric bicycles under 750 watts/20 mph from the definition of a motor vehicle only "For purposes of motor vehicle safety standards...",