Right of way in public Street

The homeowner would definitely NOT be responsible for this: they did not cause the gravel to be dropped there, were not at fault for or were the cause of its location, and the mere fact they hired a contractor does not make them liable for what the contractor does.
The contractor *may* be liable; it depends on the balance of fault in this situation, which is a subjective determination based on the exact circumstances. If they were at fault and your wife was not,  they should be liable for the full amount of damage; if your wife was at fault and they were not, they would not be liable at all; if they and your wife were both at fault, they'd be liable for a share of the damage more-or-less proportionate to their degree or relative percentage of fault.
Here are some examples of how fault could play out:
1) A small pile (say 3' or 4' diameter) is dropped near the curb, where cars might normally park: they would not be at fault, since your wife should not be driving in the "breakdown" or "parking" lane anyway, and this is appropriate location to deposit something; your wife would be at fault.
2) The gravel is dropped in the travel lane or is so large a pile that even if dropped near the curb, it protrudes into the travel lane, and does not leave a safe zone to get around it: if your wife was driving carefully and within the speed limit, the contractor should be solely at fault; but if your wife were speeding or driving distractedly (e.g. texting or on a handheld phone) at the time, she would likely be at fault, too, since she would have been driving carelessly.