Charlie,
Sorry to hear the bad news! I can't remember anything in the Mass General Laws about space requirements for passing being so specific. 400 feet is what, ~25 car lengths? Unless you're driving when everyone else is asleep you'll rarely find that amount of free space on a road in greater Boston...
I fought
a parking ticket in Boston once, and despite my having the pictures, measurements, etc. to back up my case I still ended up stuck with paying it -- a real 'catch 22' experience, as the 'hearing' is with a city employee. Plus, I lost a vacation day to the effort.
As I understand it, the actual policeman that wrote the ticket no longer has to show up at court in MA anymore (any representative can cover), so the old 'he didn't show' reprieve apparently can't be counted on anymore. If you're up to it though, you should fight it as any ticket brings point$ on your insurance
.
Best of luck! I guess I'll be watching more carefully now when passing through Acton!
-Don