Happy Truck

I'm driving on my 40 m.p.h. road with a black Toyota SUV on my tail. I rarely have too much trouble on that road because the limit is 40, but it seems like it could be less. Then, I turn onto my 30 m.p.h stretch and the lady in the SUV turns as well. Now, I know I'm in for some serious tailgating. But, wait, what's that up ahead? It's a big dump truck averaging 25-28. I just get a big smile on my face because now I don't have to worry about annoying the lady in the SUV. It's not my fault; I can't go any faster.