When the premium is not mandated by the government, it is usually derived from the calculations of an actuary, based on statistical data. The premium can vary depending on many factors that are believed to affect the expected cost of future claims. Those factors can include the car characteristics, the coverage selected (deductible, limit, covered perils), the profile of the driver (age, gender, driving history) and the usage of the car (commute to work or not, predicted annual distance driven).
Liability insurance pays for damage to someone else’s property or for injury to other persons resulting from an accident for which the insured is judged legally liable; collision insurance pays for damage to the insured car if it collides with another vehicle or object; comprehensive insurance pays for damage to the insured car resulting from fire or theft or many other causes; medical-payment insurance covers medical treatment for the policyholder and his passengers.
Widespread use of the motor car began after the First World War in urban areas. Cars were relatively fast and dangerous by that stage, yet there was still no compulsory form of car insurance anywhere in the world. This meant that injured victims would seldom get any compensation in an accident, and drivers often faced considerable costs for damage to their car and property.
In 1998, the Progressive Insurance company started a pilot program in Texas, in which drivers received a discount for installing a GPS-based device that tracked their driving behavior and reported the results via cellular phone to the company. The program was discontinued in 2000. In following years many policies (including Progressive) have been trialed and successfully introduced worldwide into what are referred to as Telematic Insurance. Such 'telematic' policies typically are based on black-box insurance technology, such devices derive from a stolen vehicle and fleet tracking but are used for insurance purposes. Since 2010 GPS-based and Telematic Insurance systems have become more mainstream in the auto insurance market not just aimed at specialised auto-fleet markets or high value vehicles (with an emphasis on stolen vehicle recovery). Modern GPS-based systems are branded as 'PAYD' Pay As You Drive insurance policies, 'PHYD' Pay How You Drive or since 2012 Smartphone auto insurance policies which utilise smartphones as a GPS sensor, e.g. . A detailed survey of the smartphone as measurement probe for insurance telematics is provided in 
As a practical matter, resetting odometers requires equipment plus expertise that makes stealing insurance risky and uneconomical. For example, to steal 20,000 miles [32,200 km] of continuous protection while paying for only the 2000 in the 35000 to 37000 range on the odometer, the resetting would have to be done at least nine times, to keep the odometer reading within the narrow 2,000-mile [3,200 km] covered range. There are also powerful legal deterrents to this way of stealing insurance protection. Odometers have always served as the measuring device for resale value, rental and leasing charges, warranty limits, mechanical breakdown insurance, and cents-per-mile tax deductions or reimbursements for business or government travel. Odometer tampering, detected during claim processing, voids the insurance and, under decades-old state and federal law, is punishable by heavy fines and jail.
There are three states that do not have a private CTP scheme. In Victoria, the Transport Accident Commission provides CTP through a levy in the vehicle registration fee, known as the TAC charge. A similar scheme exists in Tasmania through the Motor Accidents Insurance Board. A similar scheme applies in Western Australia, through the Insurance Commission of Western Australia (ICWA).
Liability auto insurance protects you from that worst case scenario by providing a cushion between your assets and the amount you’re on the hook for. For this reason, choosing the right auto liability limits is the most important part of your car insurance quote comparison. NerdWallet typically recommends having at least as much liability coverage as your net worth.