Not the same as procedural due process, which is about fair notice and a fair hearing. The idea here is different: some government actions are unconstitutional not because the process was bad, but because the government went too far in what it tried to do. Under substantive due process, courts look at whether a law or official action unfairly interferes with fundamental rights such as bodily autonomy, parental rights, privacy, or other deeply protected liberty interests under the Fourteenth Amendment.

That matters in real life when the state makes decisions that affect a person's body, family, movement, or ability to live safely and freely. If a right is considered fundamental, the government usually must meet the demanding strict scrutiny standard. If no fundamental right is involved, courts often use the more forgiving rational basis test.

For an injury claim, substantive due process is usually not the main issue in an ordinary crash case, even on dangerous roads like US-101 where fog, curves, and landslides can make a bad situation worse. Most accident claims turn on negligence, causation, and damages. But it can matter if a person argues that a government policy, custody decision, medical restraint, or law limiting recovery violated a protected liberty interest. In those cases, the claim is not just that the government acted carelessly, but that it crossed a constitutional line.