The terms "underinsured motorist" and "uninsured motorist" are sometimes used in an interchangeable manner. They do have similar meanings, but in Pennsylvania, there are slight differences. An uninsured motorist -- "UM" in insurance parlance -- has no insurance coverage at all. An underinsured motorist (UIM), on the other hand, has insurance, but an insufficient amount to cover the claims in a car accident in which they were involved. In Pennsylvania, a driver is only required to maintain liability coverage, which may not be enough in the event of an accident.
Fortunately, drivers can protect themselves in the event of uninsured/underinsured motorist accidents. While drivers in the state are no longer required to purchase it, insurers are required to offer uninsured and underinsured motorist insurance coverage to their policyholders. This is coverage that a driver can add to their policy and invoke when they are involved in an accident with someone who does not carry enough insurance to cover the damage and injuries they caused.
An advantage to UM and UIM coverage is that it can be "stacked." This means that it can be added to other insurance coverage in an attempt to cover all the medical expenses, lost wages, and damages caused by the accident. Unfortunately, though, if a driver has not added UM/UIM coverage to their policy, it will not be available to them if they are involved in an accident with a motorist who has insufficient insurance coverage.
Accidents happen and cannot always be avoided. Drivers simply cannot know or predict whether they will be involved in an accident - or with whom. What drivers can do is protect themselves by purchasing UM/UIM coverage and adding it to their auto insurance policy.
Source: FindLaw.com, "Underinsured motorist property damage coverage and claims," accessed on Feb. 6, 2018