"She was kind of driver who stayed close to the berm, her braking light flashing on and off like Christmas bulbs"

I sometimes borrows a hardcopy of book as an accessory to the audiobook I borrow and occasionally revert to the real version of book when sometimes the reading is beyond my listening comprehension. I could not understand what "close to the berm" mean in the aformentioned quote so I opened the real book to look it up