In torts, an injured party is trying to establish negligence. Negligence is defined as a duty of care owed to the world at large, that the duty was breached, and that the individual breaching the duty caused the injury. To be fair it is a little more complicated that this, ask a personal injury lawyer for assistance in your state.
Here is the upshot of a personal injury tort case, damages owed to an injured party can be immense. This can include medical expenses, all medical expenses from surgeries to complications and medicines. It would also include damages for pain and suffering and damages for loss of work or wages.Included will be interest on the damages, depending on the state interest can accrue from the time the injury occurred. Depending on state law, attorneys fees and litigation costs may also be provided.
Here is the key. If the other party at fault, or thought to be at fault, does not provide their information and disappears into the world, then the upshot vanishes. Get the police, their report will get all the information necessary to find the individual. Get an ambulance, start documenting the injuries, it is hard to prove a concussion if there is no onsite or quick review of status. Otherwise, you bear the costs of the injury and complications. As a rule of thumb: anytime a car hits something, such as a person, a bike, a motorcycle, a house, another car, the police should be involved.