Wheels run on a pair of roller bearings, and roller bearings occasionally fail. If this happens while at highway speeds, the results can be catastrophic. Usually, there are signals to alert you ahead of time of a potential bearing problem.
The first sign of a problem is often a low rumbling sound coming from one of the wheels, which can be heard and actually felt through the steering wheel or body of the vehicle. Often the rumbling sounds increase when turning.
Low Speed Sounds
The rumble may or may not be present at low speeds. Instead, a click or scraping sound could occur with each rotation of the wheel. Turning the vehicle left or right often exacerbates or eliminates these noises.
Jack up the vehicle far enough that the suspect wheel is off the ground. Grasp the wheel and attempt to lift it up and down or back and forth. It should be tight on the axle. If it moves on the axle, either repair it where it sits or have it towed to a repair shop.
Moments before a catastrophic failure occurs, the wheel will start smoking as the metal-on-metal friction heats and burns the grease inside the wheel hub. Stop immediately.
Loss of Wheel
If you didn't notice the rumbling, didn't notice the smoke coming from the wheel and continued down the highway the bearings will crumble, fall apart inside the wheel, some parts of the metal will actually melt. The wheel will become very loose on the axle and may completely disconnect from the vehicle. If it's a front wheel, the ability to steer the vehicle will be impeded.