This is a new behavior for my high energy Shepherd girl. When out on walks, she has (on two occasions recently) tried to chase passing school buses and semi-trucks. Nope, she doesn't bother with cars or pickups - just large vehicles. Granted, it's been hard to wear off some of her rambunctiousness outside with 2+ feet of snow on the ground. How do I convince her that "bigger isn't always better"?