This is how I see the actual scenario: Driver headed north on I-95 sees car between mile marker 139 and 140. He calls in on his cell phone to 911. The dispatcher asks his location, and the driver sees the next mile marker, number 141 and says "Mile marker "141". The dispatcher sends the State Trooper there instead of back where the accident actually occurred.
Here's the whole call:
http://abclocal.go.com/wtvd/story?section=central&id=4847889