Shoppers compare auto policies on the deductible because it is the number they expect to pay. The liability limit is the number that decides whether a serious claim ends at the policy or at the policyholder.
The two numbers do different jobs
A deductible is a fixed retention applied to damage to the insured's own vehicle. Its effect is bounded, known in advance and repeated at most a few times.
A liability limit is the ceiling on what the insurer will pay other people for injury or property damage the insured caused.
One caps a small predictable cost. The other caps an unpredictable and potentially very large one.
Injury costs are not bounded by the vehicle
Property damage is limited by what a car is worth. Bodily injury is limited by medical treatment, lost earnings and, in litigation, by non-economic damages.
A single serious injury can therefore produce a claim far larger than the value of every vehicle involved in the collision.
State minimum limits were set to allow access to the road rather than to indemnify losses of that size.
Exhausted limits transfer the rest to the insured
When a judgment or settlement exceeds the policy limit, the insurer pays its limit and its duty to indemnify ends at that point.
The balance remains the responsibility of the insured, who may face wage garnishment or liens depending on the rules where the claim is brought.
Collection rules differ by jurisdiction and change over time, but the structural exposure is the same wherever the limit runs out.
Defence costs usually sit outside the limit
Personal auto policies typically pay defence costs in addition to the limit rather than out of it, which preserves the full limit for the injured claimant.
That is not universal in commercial contracts, where defence costs frequently erode the limit as the case proceeds.
The distinction matters because a defence that consumes the limit leaves less money for the injury it was defending against.
Higher limits cost less than people expect
Most of an auto premium pays for frequent, moderate claims. Very large claims are rare, so the marginal cost of a higher ceiling is small next to the first layer of cover.
Umbrella policies extend liability above the underlying limits across several policies at once, and they require those underlying limits to be set at a stated level first.
Comparing quotes at identical limits, rather than at each insurer's default, is what makes two prices comparable at all.