I actually rode this route today (not taking part in the event) and it's a really fast course. There is no way that that car can travel at such a low speed without people overtaking.
I think in the past, the car went out quite early, sort of sweeping the route ahead of the ride. It wasn't like it rolled out at the front of the pack. Obviously it may have changed; they might want to stop it being a race for the first wave or two.
I actually rode this route today (not taking part in the event) and it's a really fast course. There is no way that that car can travel at such a low speed without people overtaking.