A recent story on The New York Times' Well blog tries to answer that question, although the results are fairly confusing. For example, there's no commonly accepted measure of when it's safe to get behind the wheel after a broken bone. While this is largely because every injury and more importantly, every patient, is different, the lack of clarity can be confusing for the average driver.
Making matters worse is that litigation-fearing doctors mistakenly believe they can be sued by "clearing" a patient to drive, although the NYT reports that insurance companies and police place responsibility for any post-op driving accidents on the head of the patient.
The entire piece is well worth a read, particularly if you're recovering from an injury. Hop over to The New York Times Well blog for the full story.