Yes with many different runs available the hours can be flexible
It's all over the place. Some days they are flexible, other days they will force extras.
They can also change their minds on vacation days they approved, and tell you they are revoking those days you had been told were good to take off. So if you had plans set, you will have to cancel or call in sick. If you call in sick (cuz you had plans obviously), you will be penalized.
As flexible as any trucking job.. if looking for truly flexible time scales I suggest a different line of work