The board of supervisors in each county or city and county, having a population of one million or more, may purchase ambulances, establish and maintain an ambulance service, and prescribe rules for the government and management thereof. In any county where such a service has been established, any person who has been injured in an accident or is ill and in need of immediate transportation to a hospital may be taken to any available hospital. If he is indigent and unable to pay for the service, the cost shall be a proper charge against the county. If he is not indigent, he shall reimburse the county for the cost of transportation, which shall be in accordance with a schedule to be adopted by the board, and in no case less than the actual cost.