'Goliath' Scenario issue

I've played the 'Goliath' scenario over a half-dozen times or so, and every time the truck gets to the tunnel, it doesn't slow down and crashes into one of the median support beams. That makes this scenario almost impossible to fail/lose, just so long as you don't crash into or fall too far behind the truck you're chasing. I know that this is not how the scenario is supposed to go, because I have played it on previous versions, where this doesn't happen.