Sometimes the debris on a roadway will cause or contribute to an accident or injury.
The most common ways that road debris contributes to accidents is by either causing a driver to lose control of his vehicle, or by being launched by the tires of a car into the windshield of another vehicle. If items in the roadway are particularly large, they can also necessitate sudden braking or require drivers to change lanes, increasing the risk of a collision.
Most drivers have personal experience with having a stone or object striking their windshield - it can be very surprising and distracting. In some cases the object will go through the car window, and even strike the driver or another occupant.
Some roadways are susceptible to falling rock or to mudslides. If you are driving in an area which appears potentially susceptible to fallen rock or mud on the road, or where signs indicate that you should watch for rocks on the road, you should modify your driving conduct accordingly.
The government will typically attend to the presence of road debris, by periodically cleaning the shoulder or responding to remove large items of debris from the road, and it is usually difficult to establish any governmental liability arising from the presence of debris on the road. Typically you must establish both that the government had knowledge of its presence, and that they failed to respond to its presence within a reasonable amount of time. Governmental immunity can provide an additional defense.
On the other hand, it is often difficult to identify the driver who is responsible for the presence of the debris. Arriving even a minute after a truck loses a piece of retread, or a muffler falls off of a car, it can be near impossible for a driver to establish how the debris came to be on the road. It can thus be very difficult to recover damages for injuries caused by road debris.
Sometimes the debris will be seen to fall off of the back of a truck or other vehicle, in which case it may be possible to pursue a negligence claim against the driver for the harm caused by the debris.