Motorists had to suffer through a 45-mile back-up on the Massachusetts Turnpike Friday in one of the biggest traffic jams on the highway in years. The state’s Department of Transportation warned drivers to expect delays because of the backup, which was from Charlton to Boston running through Exit 19 to Exit 9. David Procopio, State Police spokesman, said despite some car crashes and disabled vehicles throughout the day, the high volume of traffic was due to people traveling for the holiday weekend. “It’s one of the heaviest traffic days we’ve seen this year,” Procopio added.