In relation to trucking a bobtail is a tractor without a trailer attached. The term is linked to the definition of an animal with a short or non-existent tail. A tractor sans trailer has no tail a'tall.
One thing to consider when following a bobtail is that it cannot stop as well on wet pavement as a tractor trailer. The lighter weight coupled with the shorter wheel base makes it inherently less stable than a full tractor trailer. Also, the brakes on the tractor are designed to help stop a rig weighing 80,000 pounds. The tractor alone weighs about 18,000 pounds. The brakes will tend to 'lock up', causing the tractor to slide when severe braking is applied.